Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_asn1write.function b/tests/suites/test_suite_asn1write.function
index 469b971..d332485 100644
--- a/tests/suites/test_suite_asn1write.function
+++ b/tests/suites/test_suite_asn1write.function
@@ -12,7 +12,7 @@
     size_t size;
 } generic_write_data_t;
 
-int generic_write_start_step(generic_write_data_t *data)
+static int generic_write_start_step(generic_write_data_t *data)
 {
     mbedtls_test_set_step(data->size);
     mbedtls_free(data->output);
@@ -26,7 +26,7 @@
     return 0;
 }
 
-int generic_write_finish_step(generic_write_data_t *data,
+static int generic_write_finish_step(generic_write_data_t *data,
                               const data_t *expected, int ret)
 {
     int ok = 0;