put classes related to ncurses into NCurses namespace

src/ncmpcpp.h

@@ -33,6 +33,8 @@
 				vFoundPositions.clear(); \
 			} while (0)
 
+using namespace NCurses;
+
 typedef std::pair<std::string, std::string> string_pair;
 
 const int ncmpcpp_window_timeout = 500;