Suppress harmless warning in a standard header with VC14

imagehlp.h from 8.1 SDK contains an extraneous "typedef" in an enum
declaration which results in several warnings when building, avoid them.

(cherry picked from commit fbe7fc85b2)

include/wx/msw/debughlp.h

@@ -15,7 +15,20 @@
 
 #include "wx/msw/wrapwin.h"
 #ifndef __WXWINCE__
+#ifdef __VISUALC__
+    // Disable a warning that we can do nothing about: we get it at least for
+    // imagehlp.h from 8.1 Windows kit when using VC14.
+    #pragma warning(push)
+
+    // 'typedef ': ignored on left of '' when no variable is declared
+    #pragma warning(disable:4091)
+#endif
+
 #include <imagehlp.h>
+
+#ifdef __VISUALC__
+  #pragma warning(pop)
+#endif
 #endif // __WXWINCE__
 #include "wx/msw/private.h"