ProxyDatabasePlugin: new database plugin using libmpdclient

Implementation incomplete, but sort-of-works.  DumpDatabase works, but
MPD is still hard-coded on the "simple" plugin.

src/DatabaseRegistry.cxx

@@ -17,13 +17,18 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include "config.h"
 #include "DatabaseRegistry.hxx"
 #include "db/SimpleDatabasePlugin.hxx"
+#include "db/ProxyDatabasePlugin.hxx"
 
 #include <string.h>
 
 const DatabasePlugin *const database_plugins[] = {
 	&simple_db_plugin,
+#ifdef HAVE_LIBMPDCLIENT
+	&proxy_db_plugin,
+#endif
 	NULL
 };