Eliminate -Wzero-as-null-pointer-constant warnings in the headers

include/wx/utils.h

@@ -840,7 +840,7 @@ WXDLLIMPEXP_CORE bool wxYieldIfNeeded();
                        size_t *outLen,
                        const wxString& resourceName,
                        const wxChar* resourceType = wxUserResourceStr,
-                       WXHINSTANCE module = 0);
+                       WXHINSTANCE module = NULL);
 
     // This function allocates a new buffer and makes a copy of the resource
     // data, remember to delete[] the buffer. And avoid using it entirely if
@@ -851,7 +851,7 @@ WXDLLIMPEXP_CORE bool wxYieldIfNeeded();
     wxLoadUserResource(const wxString& resourceName,
                        const wxChar* resourceType = wxUserResourceStr,
                        int* pLen = NULL,
-                       WXHINSTANCE module = 0);
+                       WXHINSTANCE module = NULL);
 #endif // __WINDOWS__
 
 #endif