anv/android: fix missing dependencies issue during parallel build

The libmesa_anv_gen* modules require anv_extensions.h, patch makes sure
it gets generated as a dependency before building them.

Signed-off-by: Chenglei Ren <chenglei.ren@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 13b38ca1)