Not all prophets have recorded prophecy
It should be noted that it is possible to a prophet and not have any of your prophetic utterances recorded in scripture, for example in 1 Kings 18:4 we read "For so it was, while Jezebel massacred the prophets of the LORD, that Obadiah had taken one hundred prophets and hidden them, fifty to a cave, and had fed them with bread and water." Scripture does not contain a single word of prophecy these men uttered, but scripture still identifies them as prophets. Just because Abel does not have any words of prophecy attributed to him that does not mean he was not a prophet.
Jesus' intention in identifying Abel
Jesus is identifying Abel as the first martyr and Zechariah as the last Old Testament martyr. In that context he identifies them both as 'prophets.'
The word "prophet"
The word prophet is προφήτης. Louw Nida has an interesting entry for this term
There is a tendency in a number of languages to translate προφήτης
only in the sense of ‘one who foretells the future,’ but foretelling
the future was only a relatively minor aspect of the prophet’s
function, though gradually it became more important. Patristic authors
defined the function of a prophet mainly in terms of foretelling the
future. In New Testament times, however, the focus was upon the
inspired utterance proclaimed on behalf of and on the authority of
God. Accordingly, in a number of languages it is more appropriate to
translate προφήτης as ‘one who speaks for God.’
Louw, J. P., & Nida, E. A. (1996). Greek-English lexicon of the New
Testament: based on semantic domains (electronic ed. of the 2nd
edition., Vol. 1, p. 542). New York: United Bible Societies.
In the LXX in places like Ex 7:1 this word is used in the sense of being a spokesperson, Exodus 7:1 καὶ εἶπεν κύριος πρὸς Μωυσῆν λέγων ἰδοὺ δέδωκά σε θεὸν Φαραω καὶ Ααρων ὁ ἀδελφός σου ἔσται σου προφήτης
This is a translation of Hebrew word נָבִיא which translates into English as spokesman, speaker, or prophet (see Brown Driver Briggs) and it is a term used of man like Abraham (Gen 20:7) as well as men like Nathan (1 Kings 1:8).
Neither Abraham or Aaron are presented in scripture as 'prophets' in the specific sense we usually think of, i.e. one who foretells and 'forthtells' God's word yet they are both God's spokesman in that they both has a testimony to the world around them.
The testimony of Abel
Abel did have a testimony before men. He had a testimony through his sacrifice that was accepted by God, Gen 4:4 which ultimately led to his death at the hands of his brother (Gen 4:8).
That testimony persisted long after his death, and continues today through the record of scripture.
Hebrews 11:4 "By faith Abel offered to God a more excellent sacrifice
than Cain, through which he obtained witness that he was righteous,
God testifying of his gifts; and through it he being dead still
speaks."
It is through God's acceptance of Abel's sacrifice that he continues to speak to every generation of those who read God's word.
Conclusion
Abel may or may not have spoken words of prophecy that were given him by direct revelation from God, but even after death his testimony still speaks to people. In that sense he is God's spokesperson and the words נָבִיא and προφήτης as they are used in scripture are elastic enough in their meanings to be used to describe that type of person.
