UI: Don't influence headers with using namespace

Preparation for C++17 compilation.

UI/platform-windows.cpp

@@ -20,7 +20,6 @@
 #include "obs-config.h"
 #include "obs-app.hpp"
 #include "platform.hpp"
-using namespace std;
 
 #include <util/windows/win-version.h>
 #include <util/platform.h>
@@ -37,6 +36,8 @@ using namespace std;
 #include <util/windows/HRError.hpp>
 #include <util/windows/ComPtr.hpp>
 
+using namespace std;
+
 static inline bool check_path(const char *data, const char *path,
 			      string &output)
 {