Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index 70e7bad..878ceed 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -9,7 +9,7 @@
     char *ptr;
 };
 
-void string_debug(void *data, int level, const char *file, int line, const char *str)
+static void string_debug(void *data, int level, const char *file, int line, const char *str)
 {
     struct buffer_data *buffer = (struct buffer_data *) data;
     char *p = buffer->ptr;