Remove all using directives for STL namespace and members

Made all STL usages explicit to be able automatically find all usages of
particular class or function.

samples/gpu/brox_optical_flow.cpp

@@ -1,6 +1,7 @@
 #include <iostream>
 #include <iomanip>
 #include <string>
+#include <cctype>
 
 #include "cvconfig.h"
 #include "opencv2/core/core.hpp"