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/msgb/msgb_test.c b/tests/msgb/msgb_test.c
index bd52f9f..7966103 100644
--- a/tests/msgb/msgb_test.c
+++ b/tests/msgb/msgb_test.c
@@ -61,7 +61,7 @@
 	return *exception == 0;
 }
 
-static void test_msgb_api()
+static void test_msgb_api(void)
 {
 	struct msgb *msg = msgb_alloc_headroom(4096, 128, "data");
 	unsigned char *cptr = NULL;
@@ -117,7 +117,7 @@
 	msgb_free(msg);
 }
 
-static void test_msgb_api_errors()
+static void test_msgb_api_errors(void)
 {
 	struct msgb *msg = msgb_alloc_headroom(4096, 128, "data");
 	volatile int e = 0;
@@ -138,7 +138,7 @@
 	osmo_set_panic_handler(NULL);
 }
 
-static void test_msgb_copy()
+static void test_msgb_copy(void)
 {
 	struct msgb *msg = msgb_alloc_headroom(4096, 128, "data");
 	struct msgb *msg2;
@@ -185,7 +185,7 @@
 	msgb_free(msg2);
 }
 
-static void test_msgb_resize_area()
+static void test_msgb_resize_area(void)
 {
 	struct msgb *msg = msgb_alloc_headroom(4096, 128, "data");
 	int rc;
@@ -285,7 +285,7 @@
 	osmo_set_panic_handler(NULL);
 }
 
-static void test_msgb_printf()
+static void test_msgb_printf(void)
 {
 	struct msgb *msg;
 	struct msgb *msg_ref;