WinRT winmain: fix linker error on x86 compilers

The x86 PC and Phone emulator compilers choke on WinMain's signature not
matching the stdcall exported version. This fixes the issue.

Change-Id: I30d8a5dab67f3f1f15869abe2928326e3401dc43
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>

src/winmain/qtmain_winrt.cpp

@@ -159,7 +159,7 @@ private:
 };
 
 // Main entry point for Appx containers
-int WinMain()
+int __stdcall WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
 {
     int argc = 0;
     char **argv, **env;