package/mesa3d: add option to configure gbm support

Kodi 18.0-Leia implements stand-alone gbm support alongside x11 &
wayland. To enable building gbm support in mesa3d without x11 &
wayland we need to add a specific configure option for it.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>