xf86drm.c: fix build failure

./xf86drm.c: In function 'drmNodeIsDRM':
../xf86drm.c:2825:7: error: "__FreeBSD__" is not defined [-Werror=undef]
 #elif __FreeBSD__
       ^
../xf86drm.c: In function 'drmGetMinorNameForFD':
../xf86drm.c:2938:7: error: "__FreeBSD__" is not defined [-Werror=undef]
 #elif __FreeBSD__
       ^
../xf86drm.c: In function 'drmParsePciBusInfo':
../xf86drm.c:3258:7: error: "__FreeBSD__" is not defined [-Werror=undef]
 #elif __FreeBSD__
       ^
../xf86drm.c: In function 'drmParsePciDeviceInfo':
../xf86drm.c:3427:7: error: "__FreeBSD__" is not defined [-Werror=undef]
 #elif __FreeBSD__
       ^
../xf86drm.c: In function 'drmGetDeviceNameFromFd2':
../xf86drm.c:4305:7: error: "__FreeBSD__" is not defined [-Werror=undef]
 #elif __FreeBSD__
       ^

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>