fix compile error on 32bit systems

4c2766b6 (drm_mmap/drm_unmap) brought this error for every .c file that
was not #including config.h:

  In file included from private.h:4:0,
                   from abi16.c:29:
  ../libdrm.h: In function 'drm_munmap':
  ../libdrm.h:81:4: error: size of unnamed array is negative

Signed-off-by: Rob Clark <robdclark@gmail.com>