Why did Jesus call Abel a prophet? In Luke 11:50-51, the Lord Jesus Christ calls Abel a prophet:


  "Therefore this generation will be held responsible for the blood of
  all the prophets that has been shed since the beginning of the world,
  from the blood of Abel to the blood of Zechariah, who was killed
  between the altar and the sanctuary. Yes, I tell you, this generation
  will be held responsible for it all." Luke 11:50-51 NIV


I have in mind the concept of prophet as one who acted as God's mouthpiece, relaying the words of God to men in accordance with God's instructions.
However, I coun't find no record of any prophetic utterance from Abel.

What then made him a prophet?
The broadest sense of an OT prophet was the Spirit of God coming up or moving a person to reveal something about God.  The narrow sense is a prophet speaking "thus sayeth the LORD".  It is in the former sense that Christ calls Abel a prophet.


  And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.  And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: ... If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
  Gen 4:3-4,7
  
  Prophet:
  "one who speaks forth or openly" (see PROPHECY, A), "a proclaimer of a divine message," 
  Vines


So, how was Abel a prophet?  What was the divine message from God as regards the offerings of Cain and Abel?


  Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.
  Gen 3:21


Obviously from the very first example of God slaying the animal to cover the sin of Adam and Eve, the prophetic voice of Abel is one of blood sacrifice that culminates in Christ Jesus.