xfree86: Silence a new glibc warning

glibc would like to stop declaring major()/minor() macros in
<sys/types.h> because that header gets included absolutely everywhere
and unix device major/minor is perhaps usually not what's expected. Fair
enough. If one includes <sys/sysmacros.h> as well then glibc knows we
meant it and doesn't warn, so do that if it exists.

Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit d732c365)