Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_lmots.function b/tests/suites/test_suite_lmots.function
index 293287a..61f2e28 100644
--- a/tests/suites/test_suite_lmots.function
+++ b/tests/suites/test_suite_lmots.function
@@ -3,7 +3,7 @@
 #include "mbedtls/lms.h"
 
 #if defined(MBEDTLS_TEST_HOOKS)
-int check_lmots_private_key_for_leak(unsigned char *sig)
+static int check_lmots_private_key_for_leak(unsigned char *sig)
 {
     size_t idx;