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

src/modules/whitelist/Makefile.am.include

@@ -5,6 +5,7 @@ noinst_HEADERS += src/modules/whitelist/tests_impl.h
 if USE_BENCHMARK
 noinst_PROGRAMS += bench_whitelist
 bench_whitelist_SOURCES = src/bench_whitelist.c
+bench_whitelist_CPPFLAGS = -DSECP256K1_BUILD $(SECP_INCLUDES)
 bench_whitelist_LDADD = libsecp256k1.la $(SECP_LIBS)
 bench_generator_LDFLAGS = -static
 endif