Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index f3a64e1..22a637a 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -44,7 +44,7 @@
  * test) are stored in the data member of the state structure. Each number is in
  * the format that mbedtls_mpi_read_string understands and is chunk_len long.
  */
-int mbedtls_test_mpi_miller_rabin_determinizer(void *state,
+static int mbedtls_test_mpi_miller_rabin_determinizer(void *state,
                                                unsigned char *buf,
                                                size_t len)
 {