android: pan/bi: add bi_opt_constant_fold.c to Makefile.sources

android: pan/bi: add bi_opt_constant_fold.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: bi_opt_constant_fold
>>> referenced by bifrost_compile.c:3105 (external/mesa/src/panfrost/bifrost/bifrost_compile.c:3105)

Fixes: 1cb11969bec0 ("pan/bi: Add simple constant folding pass")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10603>