libepoxy: convert to git

Latest version doesn't come with stable tarballs.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 4b6eed2b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>