UI: Use platform.h for snprintf

This allows snprintf to be used in any file in the UI.

obs/obs-app.cpp

@@ -38,9 +38,6 @@
 
 #ifdef _WIN32
 #include <windows.h>
-#if _MSC_VER < 1900
-#define snprintf _snprintf
-#endif
 #else
 #include <signal.h>
 #endif