Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_pkcs7.function b/tests/suites/test_suite_pkcs7.function
index 4c8bf23..e5dc4bd 100644
--- a/tests/suites/test_suite_pkcs7.function
+++ b/tests/suites/test_suite_pkcs7.function
@@ -17,7 +17,7 @@
  * END_DEPENDENCIES
  */
 /* BEGIN_SUITE_HELPERS */
-int pkcs7_parse_buffer(unsigned char *pkcs7_buf, int buflen)
+static int pkcs7_parse_buffer(unsigned char *pkcs7_buf, int buflen)
 {
     int res;
     mbedtls_pkcs7 pkcs7;