Android: generate symlinks for all enabled gallium drivers

Android: generate symlinks for all enabled gallium drivers

Current post install command relies on GALLIUM_TARGET_DRIVERS variable,
however variable needs to be initialized in src/gallium/Android.mk
in order that all enabled gallium drivers symlinks are correctly generated.

At the moment due to sorting of INC_DIRS and variable set with svga (vmwgfx)
only vmwgfx_dri.so and virtio_gpu_dri.so symlinks are generated.

Fixes: a3d98ca62f ("Android: use symlinks for driver loading")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>