Fix optionally safe hooks declarations

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index 150bc76..463f145 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -746,6 +746,11 @@
     }
 }
 
+#if defined(MBEDTLS_TEST_HOOKS)
+// Set to a default that is neither MBEDTLS_MPI_IS_PUBLIC nor MBEDTLS_MPI_IS_SECRET
+int mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_PUBLIC + MBEDTLS_MPI_IS_SECRET + 1;
+#endif
+
 /*
  * This function calculates the indices of the exponent where the exponentiation algorithm should
  * start processing.