egl/main: replace INLINE with inline

egl/main: replace INLINE with inline

Drop the custom keyword in favour of the C99 one. All the places using
it now directly include c99_compat.h which should handle things on
platforms which lack it.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>