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/fsm/fsm_test.c b/tests/fsm/fsm_test.c
index 6fa0ae7..960042c 100644
--- a/tests/fsm/fsm_test.c
+++ b/tests/fsm/fsm_test.c
@@ -172,7 +172,7 @@
 	return fi;
 }
 
-static void test_id_api()
+static void test_id_api(void)
 {
 	struct osmo_fsm_inst *fi;
 
@@ -280,7 +280,7 @@
 	osmo_timers_update(); \
 } while (0)
 
-void fake_time_start()
+void fake_time_start(void)
 {
 	struct timespec *clock_override;
 
@@ -301,7 +301,7 @@
 	return 0;
 }
 
-static void test_state_chg_keep_timer()
+static void test_state_chg_keep_timer(void)
 {
 	struct osmo_fsm_inst *fi;
 
@@ -349,7 +349,7 @@
 	fprintf(stderr, "--- %s() done\n", __func__);
 }
 
-static void test_state_chg_T()
+static void test_state_chg_T(void)
 {
 	struct osmo_fsm_inst *fi;