kms: include stddef.h before drm.h

Not doing so leads to the following error, if stddef.h is not included
indirectly through EGL headers:

| libdrm/drm.h:132:2: error: unknown type name 'size_t'
|   size_t name_len;   /**< Length of name buffer */

Signed-off-by: Andreas Oberritter <obi@saftware.de>

Reviewed-by: Robert Bragg <robert@linux.intel.com>