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>
(cherry picked from commit 55c82476a93366a3e7d1a2537fccc3a7aab87c66)
