build: add -DSECP256K1_BUILD to benchmark_internal build flags

gcc 6 will warn about our non-null checks when SECP256K1_BUILD
our NONNULL marker is nontrivial. This occurs unless SECP256K1_BUILD
is set, which we had forgotten to do for the internal benchmarks,
which compile directly against the library instead of linking.

Makefile.am

@@ -84,7 +84,7 @@ bench_sign_SOURCES = src/bench_sign.c
 bench_sign_LDADD = libsecp256k1.la $(SECP_LIBS) $(SECP_TEST_LIBS) $(COMMON_LIB)
 bench_internal_SOURCES = src/bench_internal.c
 bench_internal_LDADD = $(SECP_LIBS) $(COMMON_LIB)
-bench_internal_CPPFLAGS = $(SECP_INCLUDES)
+bench_internal_CPPFLAGS = -DSECP256K1_BUILD $(SECP_INCLUDES)
 endif
 
 if USE_TESTS