plugin-loader: Fix hashing of registered plugin features

This strangely never caused any noticeable issues, but was the reason for
build failures in certain test cases (mostly BLISS) due to missing plugin
features when built with specific options on Travis (was not reproducible
locally).

src/libstrongswan/plugins/plugin_loader.c

@@ -107,7 +107,7 @@ struct registered_feature_t {
 /**
  * Hash a registered feature
  */
-static bool registered_feature_hash(registered_feature_t *this)
+static u_int registered_feature_hash(registered_feature_t *this)
 {
 	return plugin_feature_hash(this->feature);
 }