On Solaris, #include <sys/mkdev.h> in xf86drm.c

Needed on Solaris for the definitions of major() & minor() used in
drmGetNodeTypeFromFd() and makedev() used in drmOpenMinor()

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>