Fixes the following building error:
FAILED: out/target/product/x86_64/gen/STATIC_LIBRARIES/libpanfrost_bifrost_intermediates/bifrost_nir_algebraic.c
...
/usr/bin/python3: can't open file 'external/mesa/src/panfrost/bifrost/bifrost_nir_algebraic.py': [Errno 2] No such file or directory
Fixes: a67347d49637 ("pan/bi: Use nir_lower_to_bit_size")