skins2(Linux): check if _NET_WM_PID is supported

This kills X11 warnings for Ubuntu/Gnome.
(cherry picked from commit 2eb55c6d8e2c2932a9703070fddabca6a18633e0)

modules/gui/skins2/x11/x11_window.cpp

@@ -184,9 +184,12 @@ X11Window::X11Window( intf_thread_t *pIntf, GenericWindow &rWindow,
     delete[] hostname;
 
     // initialize EWMH pid
+    if( NET_WM_PID != None )
+    {
         pid_t pid = getpid();
         XChangeProperty( XDISPLAY, m_wnd, NET_WM_PID, XA_CARDINAL, 32,
                          PropModeReplace, (unsigned char *)&pid, 1 );
+    }
 
 }