build: add posix forward decl

Makefile.am

@@ -287,6 +287,7 @@ POSIX_FILES = 			\
     io_posix.ipp		\
     library_posix.hpp		\
     library_posix.cpp		\
+    posix/fwd.hpp		\
     posix/dir.cpp		\
     posix/dir.hpp		\
     posix/dir.ipp		\

posix/fwd.hpp

@@ -0,0 +1,26 @@
+/*
+ * 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
+ *
+ * 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.
+ *
+ * Copyright 2016 Danny Robson <danny@nerdcruft.net>
+ */
+
+#ifndef __CRUFT_UTIL_POSIX_FWD_HPP
+#define __CRUFT_UTIL_POSIX_FWD_HPP
+
+namespace util::posix {
+    class fd;
+    class dir;
+    class map;
+}
+
+#endif