Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_dhm.function b/tests/suites/test_suite_dhm.function
index 2090594..a7b4b40 100644
--- a/tests/suites/test_suite_dhm.function
+++ b/tests/suites/test_suite_dhm.function
@@ -1,7 +1,7 @@
 /* BEGIN_HEADER */
 #include "mbedtls/dhm.h"
 
-int check_get_value(const mbedtls_dhm_context *ctx,
+static int check_get_value(const mbedtls_dhm_context *ctx,
                     mbedtls_dhm_parameter param,
                     const mbedtls_mpi *expected)
 {