xf86drmMode.h: inline -> __inline for use with gcc -std=c89 -pedantic

Unfortunately, there are some users of libdrm installed headers that like
to be built with -std=c89 -pedantic, which does not like "inline".

However, __inline works.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>