Fix optionally safe hooks declarations

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 50c53e6..ff6360b 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -818,9 +818,9 @@
                                     mbedtls_mpi_uint *T);
 
 #if defined(MBEDTLS_TEST_HOOKS)
-int mbedtls_mpi_optionally_safe_codepath;
+extern int mbedtls_mpi_optionally_safe_codepath;
 
-static inline void mbedtls_mpi_optionally_safe_codepath_reset()
+static inline void mbedtls_mpi_optionally_safe_codepath_reset(void)
 {
     // Set to a default that is neither MBEDTLS_MPI_IS_PUBLIC nor MBEDTLS_MPI_IS_SECRET
     mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_PUBLIC + MBEDTLS_MPI_IS_SECRET + 1;