output/raop: fallback for g_set_error_literal()

This function was added in GLib 2.18.  Make the plugin compatible with
older GLib versions.

src/output/raop_output_plugin.c

@@ -20,6 +20,7 @@
 #include "output_api.h"
 #include "mixer_list.h"
 #include "raop_output_plugin.h"
+#include "glib_compat.h"
 
 #include <glib.h>
 #include <unistd.h>