osmo-arfcn: Return something from the method

This is a fatal error when building RPMs for OpenSUSE.
diff --git a/utils/osmo-arfcn.c b/utils/osmo-arfcn.c
index 2be3958..5103c97 100644
--- a/utils/osmo-arfcn.c
+++ b/utils/osmo-arfcn.c
@@ -77,6 +77,7 @@
 	printf("%s: ARFCN %4d\n",
 		gsm_band_name(gsm_arfcn2band(arfcn)),
 		arfcn & ~ARFCN_FLAG_MASK);
+	return 0;
 }
 
 static void help(const char *progname)