Add forward declaration header

Author: Levi-Armstrong

include/boost_plugin_loader/fwd.h

@@ -0,0 +1,27 @@
+/**
+ *
+ * @copyright Copyright (c) 2021, Southwest Research Institute
+ *
+ * @par License
+ * Software License Agreement (Apache License)
+ * @par
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * @par
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef BOOST_PLUGIN_LOADER_FWD_H
+#define BOOST_PLUGIN_LOADER_FWD_H
+
+namespace boost_plugin_loader
+{
+class PluginLoader;
+}
+
+#endif  // BOOST_PLUGIN_LOADER_FWD_H