Fix harmless warning about NULL in wxStyledTextCtrl code.

Cast NULL to the type of third argument of SendMsg() explicitly to avoid a
warning about it from Ubuntu g++ 4.6.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73478 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/stc/stc.cpp

@@ -4509,7 +4509,7 @@ bool wxStyledTextCtrl::GetUseAntiAliasing() {
 }
 
 void wxStyledTextCtrl::AnnotationClearLine(int line) {
-    SendMsg(SCI_ANNOTATIONSETTEXT, line, NULL);
+    SendMsg(SCI_ANNOTATIONSETTEXT, line, (sptr_t)NULL);
 }