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 <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
CC: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>
Signed-off-by: Eric Engestrom <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@engestrom.ch>
Signed-off-by: Rob Clark <915c10c999604870200b6defbe633d857c856ca0@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler