Declare res_init() for builds against Android's Bionic

res_init() exists in the C library here and as such is detected
by configure, but it is not declared in any header.

gio/gresolver.c

@@ -232,6 +232,10 @@ g_resolver_set_default (GResolver *resolver)
   default_resolver = g_object_ref (resolver);
 }
 
+/* Bionic has res_init() but it's not in any header */
+#ifdef __BIONIC__
+int res_init (void);
+#endif
 
 static void
 g_resolver_maybe_reload (GResolver *resolver)