mesa-gl: make mesa-gl really openGL-only

mesa-gl: make mesa-gl really openGL-only

The ??= operator for PACKAGECONFIG doesn't actually do anything because
the recipe includes mesa.inc which already sets this variable (with the
= operator).

This probably wasn't noticed until now because mesa-gl is likely only
ever built in its target flavor which was already set correctly thanks
to the :class-target override.

This essentially only make mesa-gl-native and nativesdk-mesa-gl follow
the same configuration as the target.

Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>