get_reviewer.pl: fix mesa check

get_reviewer.pl: fix mesa check

This script was broken for the last few days and I couldn't figure out why.
Turns out it was checking for the existence of a file that got renamed,
so rename it in here too.

Fixes: f926cf5bd0ade3273b32 ("docs: Rename GL3.txt to features.txt")
CC: Ian Romanick <ian.d.romanick@intel.com>
CC: Rob Clark <robclark@freedesktop.org>
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Rob Clark <robdclark@gmail.com>