glx: Fix typos that break GLX_ARB_context_flush_control

The trailing \n are just wrong here, __glXEnableExtension wants a string
without them.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit fd0eafb1)