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: I5ca56d885b5ce4d4c9f91ffc083c05a48d1306e4
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index 5761d78..b681888 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -262,7 +262,7 @@
 	return 0;
 }
 
-static void create_payload_types()
+static void create_payload_types(void)
 {
 	PayloadType *pt;