Move postgresql backend's include files to a new db subdir.

Fix build and remove unneeded include in tag.cpp

include/backends/postgresql/db/settings.hpp

@@ -0,0 +1,34 @@
+/* Copyright 2015, 2016, Michele Santullo
+ * This file is part of "dindexer".
+ *
+ * "dindexer" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "dindexer" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "dindexer".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef idC895D12FEB324387A37FF631E0C09560
+#define idC895D12FEB324387A37FF631E0C09560
+
+#include <string>
+#include <cstdint>
+
+namespace dinbpostgres {
+	struct Settings {
+		std::string address;
+		std::string username;
+		std::string password;
+		std::string dbname;
+		uint16_t port;
+	};
+} //namespace dinbpostgres
+
+#endif