Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_common.function b/tests/suites/test_suite_common.function
index 5c5700c..c508b11 100644
--- a/tests/suites/test_suite_common.function
+++ b/tests/suites/test_suite_common.function
@@ -1,7 +1,7 @@
 /* BEGIN_HEADER */
 #include "common.h"
 
-void fill_arrays(unsigned char *a, unsigned char *b, unsigned char *r1, unsigned char *r2, size_t n)
+static void fill_arrays(unsigned char *a, unsigned char *b, unsigned char *r1, unsigned char *r2, size_t n)
 {
     for (size_t i = 0; i < n; i++) {
         a[i]  = (unsigned char) i * 3;