Use #ifdef, not #if, to check for __WXGTK26__ symbol.

This fixes gcc warning given if __WXGTK26__ is not defined (which almost never
happens nowadays but still).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67801 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/dcclient.cpp

@@ -1492,7 +1492,7 @@ void wxWindowDCImpl::DoDrawRotatedText( const wxString &text, wxCoord x, wxCoord
 
     wxCHECK_RET( IsOk(), wxT("invalid window dc") );
 
-#if __WXGTK26__
+#ifdef __WXGTK26__
     if (!gtk_check_version(2,6,0))
     {
         x = XLOG2DEV(x);