libgluonutil: add missing gluonutil_get_primary_domain() prototype

Fixes respondd on 64bit archs, as gluonutil_get_primary_domain() was
assumed to return int without the prototype.

Fixes: bcf57467 ("libgluonutil: implement gluonutil_get_primary_domain()")

package/libgluonutil/src/libgluonutil.h

@@ -54,6 +54,7 @@ struct json_object * gluonutil_wrap_and_free_string(char *str);
 
 bool gluonutil_has_domains(void);
 char * gluonutil_get_domain(void);
+char * gluonutil_get_primary_domain(void);
 struct json_object * gluonutil_load_site_config(void);
 
 #endif /* _LIBGLUON_LIBGLUON_H_ */