Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_cipher.function b/tests/suites/test_suite_cipher.function
index 8e49d2d..040c35c 100644
--- a/tests/suites/test_suite_cipher.function
+++ b/tests/suites/test_suite_cipher.function
@@ -129,7 +129,7 @@
  * return   1 if it is,
  *          0 if it isn't.
  */
-int buffer_is_all_zero(const uint8_t *buf, size_t size)
+static int buffer_is_all_zero(const uint8_t *buf, size_t size)
 {
     for (size_t i = 0; i < size; i++) {
         if (buf[i] != 0) {