Fixed bug 4102 - define _WIN32_WINNT_WIN7 if not already defined

Ozkan Sezer

The following patch defines _WIN32_WINNT_WIN7 if it is not already
defined in core/windows/SDL_windows.c,  similar to what is already
there for _WIN32_WINNT_VISTA.

src/core/windows/SDL_windows.c

@@ -31,6 +31,9 @@
 #ifndef _WIN32_WINNT_VISTA
 #define _WIN32_WINNT_VISTA  0x0600
 #endif
+#ifndef _WIN32_WINNT_WIN7
+#define _WIN32_WINNT_WIN7   0x0601
+#endif
 
 
 /* Sets an error message based on an HRESULT */