skins2: fix a small memory leak and downgrade from err to dbg

This last occurrence of MSG_ERR led to a memory leak.
In addition, avoid spamming the logs. A debug message is enough.
(cherry picked from commit 61e145ca81f83a23c188b4dfa0bb62d4fd4acb6a)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/skins2/x11/x11_display.cpp

@@ -63,7 +63,7 @@ X11Display::X11Display( intf_thread_t *pIntf ): SkinObject( pIntf ),
     m_pDisplay = XOpenDisplay( NULL );
     if( m_pDisplay == NULL )
     {
-        MSG_ERR( "Cannot open display" );
+        msg_Dbg( pIntf, "Cannot open X display" );
         return;
     }