android: nir: add nir_lower_fragcolor.c to Makefile.sources
Fixes the following building error:
FAILED: out/target/product/x86_64/obj/SHARED_LIBRARIES/gallium_dri_intermediates/LINKED/gallium_dri.so
...
ld.lld: error: undefined symbol: nir_lower_fragcolor
>>> referenced by pan_assemble.c:81 (external/mesa/src/gallium/drivers/panfrost/pan_assemble.c:81)
Cc: 21.0 21.1 <mesa-stable@lists.freedesktop.org>
Fixes: 1fd356302590 ("nir: add lowering pass for fragcolor -> fragdata")