package/intel-mediadriver: disable -Werror

Disable -Werror to avoid the following build failure:

<command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]

MEDIA_BUILD_FATAL_WARNINGS option is available since version 18.2.0 and
https://github.com/intel/media-driver/commit/6932fc0ffb8228245052528820adafb6743f7482

Fixes:
 - http://autobuild.buildroot.org/results/52638d95312e464626d1c4047b3b26d4f57a1cd2



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>