Add support for SDL_render_d3d12.c to compile in C++ mode

src/core/windows/SDL_windows.h

@@ -65,6 +65,11 @@
 #define SDL_tcsstr SDL_strstr
 #endif
 
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Sets an error message based on a given HRESULT */
 extern int WIN_SetErrorFromHRESULT(const char *prefix, HRESULT hr);
 
@@ -104,6 +109,11 @@ extern BOOL WIN_IsEqualIID(REFIID a, REFIID b);
 extern void WIN_RECTToRect(const RECT *winrect, SDL_Rect *sdlrect);
 extern void WIN_RectToRECT(const SDL_Rect *sdlrect, RECT *winrect);
 
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _INCLUDED_WINDOWS_H */
 
 /* vi: set ts=4 sw=4 expandtab: */