Fix compiler warning: no-return-in-nonvoid-function main.c

RPM post-build-checks found some issue and marks these as error:
[  31s] I: Program returns random data in a function
[  31s] E: osmo-remsim no-return-in-nonvoid-function main.c:49

Change-Id: I18705488d0ef8a445004e6a7e81dd2483afb9bdb

src/main.c

@@ -46,4 +46,5 @@ int main(int argc, char **argv)
 	g_ctx = talloc_named_const(NULL, 0, "main");
 	talloc_asn1_ctx = talloc_named_const(g_ctx, 0, "asn1_context");
 	card_readers_probe(g_ctx);
+	return 0;
 }