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/tdef/tdef_test.c b/tests/tdef/tdef_test.c
index c348e69..a0fcc9a 100644
--- a/tests/tdef/tdef_test.c
+++ b/tests/tdef/tdef_test.c
@@ -125,7 +125,7 @@
 	}
 }
 
-static void test_tdef_get_nonexisting()
+static void test_tdef_get_nonexisting(void)
 {
 	printf("\n%s()\n", __func__);
 
@@ -136,7 +136,7 @@
 	print_tdef_get(tdefs, 5, OSMO_TDEF_US);
 }
 
-static void test_tdef_set_and_get()
+static void test_tdef_set_and_get(void)
 {
 	struct osmo_tdef *t;
 	printf("\n%s()\n", __func__);
@@ -317,7 +317,7 @@
 	osmo_timers_update(); \
 } while (0)
 
-void fake_time_start()
+void fake_time_start(void)
 {
 	struct timespec *clock_override;