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: 1cb11969 ("pan/bi: Add simple constant folding pass")