Declare wxInvalidSize also in wx/dir.h

wxInvalidSize is a documented return value for wxDir::GetTotalSize(),
yet it was not available by including just wx/dir.h as it was declared
in wx/filename.h only.

Fix this by declaring it in wx/dir.h too.

Closes https://github.com/wxWidgets/wxWidgets/pull/609

interface/wx/dir.h

@@ -15,6 +15,11 @@ enum wxDirTraverseResult
     wxDIR_CONTINUE      ///< Continue into this directory.
 };
 
+/**
+    The return value of wxDir::GetTotalSize() in case of error.
+*/
+wxULongLong wxInvalidSize;
+
 /**
     @class wxDirTraverser