Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_asn1parse.function b/tests/suites/test_suite_asn1parse.function
index 01a091b..394d08a 100644
--- a/tests/suites/test_suite_asn1parse.function
+++ b/tests/suites/test_suite_asn1parse.function
@@ -122,7 +122,7 @@
     return ERR_PARSE_INCONSISTENCY;
 }
 
-int get_len_step(const data_t *input, size_t buffer_size,
+static int get_len_step(const data_t *input, size_t buffer_size,
                  size_t actual_length)
 {
     unsigned char *buf = NULL;