android: panfrost/lib: add pan_cs.c to Makefile.sources

Fixes the following building errors:

FAILED: out/target/product/x86_64/obj/SHARED_LIBRARIES/gallium_dri_intermediates/LINKED/gallium_dri.so ... ld.lld: error: undefined symbol: pan_emit_tls

referenced by pan_cmdstream.c:2279 (external/mesa/src/gallium/drivers/panfrost/pan_cmdstream.c:2279) ... ld.lld: error: undefined symbol: pan_emit_fbd referenced by pan_cmdstream.c:2302 (external/mesa/src/gallium/drivers/panfrost/pan_cmdstream.c:2302) ... ld.lld: error: undefined symbol: pan_emit_fragment_job referenced by pan_cmdstream.c:2382 (external/mesa/src/gallium/drivers/panfrost/pan_cmdstream.c:2382) ... ld.lld: error: undefined symbol: pan_emit_bifrost_tiler_heap referenced by pan_job.c:718 (external/mesa/src/gallium/drivers/panfrost/pan_job.c:718) ... ld.lld: error: undefined symbol: pan_emit_bifrost_tiler referenced by pan_job.c:723 (external/mesa/src/gallium/drivers/panfrost/pan_job.c:723) ... ld.lld: error: undefined symbol: pan_fbd_has_zs_crc_ext referenced by pan_job.c:893 (external/mesa/src/gallium/drivers/panfrost/pan_job.c:893) ... clang-9: error: linker command failed with exit code 1 (use -v to see invocation)

Cc: 21.1 mesa-stable@lists.freedesktop.org Fixes: 387f8c03 ("panfrost: Add various helpers to simplify FB desc emission")