whitelist: add SECP_INCLUDES to bench_whitelist CPPFLAGS
whitelist: add SECP_INCLUDES to bench_whitelist CPPFLAGS

This will fix the following compile error on macOS

In file included from src/num.h:17,
                 from src/num_impl.h:14,
                 from src/bench_whitelist.c:14:
src/num_gmp.h:10:10: fatal error: gmp.h: No such file or directory

ElementsProject/secp256k1-zkp,ElementsProject/secp256k1-zkp