imgui_impl_win32: Don't redefine WIN32_LEAN_AND_MEAN if already defined (#2014)

examples/imgui_impl_win32.cpp

@@ -8,7 +8,9 @@
 
 #include "imgui.h"
 #include "imgui_impl_win32.h"
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <windows.h>
 #include <tchar.h>