Fix XYToPosition() for the last position in first line in wxGTK

This is similar to e74fb5effe for wxMSW
and ensures that we don't consider the last position on the first line
invalid in wxGTK neither.

This fixes TextCtrlTestCase::XYToPositionSingleLine() unit test failure.

src/gtk/textctrl.cpp

@@ -1192,8 +1192,7 @@ long wxTextCtrl::XYToPosition(long x, long y ) const
 {
     if ( IsSingleLine() )
     {
-
-        if ( y != 0 || x >= GTKGetEntryTextLength(GTK_ENTRY(m_text)) )
+        if ( y != 0 || x > GTKGetEntryTextLength(GTK_ENTRY(m_text)) )
             return -1;
 
         return x;