Support building with -Werror=strict-prototypes / -Werror=old-style-definition

Unfortunately "-std=c99" is not sufficient to make gcc ignore code that
uses constructs of earlier C standards, which were abandoned in C99.

See https://lwn.net/ml/fedora-devel/Y1kvF35WozzGBpc8@redhat.com/ for
some related discussion.

Change-Id: I84fd99442d0cc400fa562fa33623c142649230e2
diff --git a/tests/gsm29205/gsm29205_test.c b/tests/gsm29205/gsm29205_test.c
index 5d4835b..6598f89 100644
--- a/tests/gsm29205/gsm29205_test.c
+++ b/tests/gsm29205/gsm29205_test.c
@@ -28,7 +28,7 @@
 #include <string.h>
 #include <errno.h>
 
-static void test_gcr()
+static void test_gcr(void)
 {
 	static const uint8_t res[] = {
 		0x03, /* .net_len */