Use const for constant local variables in wxWidgets headers

This avoids warnings from MSVS 2017 static analyzer when C++ core rule
set is activated.

Closes https://github.com/wxWidgets/wxWidgets/pull/819

include/wx/msw/ole/oleutils.h

@@ -38,7 +38,7 @@
 // return true if ok, false otherwise
 inline bool wxOleInitialize()
 {
-    HRESULT
+    const HRESULT
     hr = ::OleInitialize(NULL);
 
     // RPC_E_CHANGED_MODE indicates that OLE had been already initialized