Fix warning from double assigning to gscreen

Compiling GParted on an older distribution with gtk+-3.0 < 3.22.0, where
HAVE_GTK_SHOW_URI_ON_WINDOW is undefined, produces this warning message:
    Win_GParted.cc: In member function 'void GParted::Win_GParted::show_help(const Glib::ustring&, const Glib::ustring&)':
    Win_GParted.cc:1822:56: warning: operation on 'gscreen' may be undefined [-Wsequence-point]
      GdkScreen *gscreen = gscreen = gdk_screen_get_default();
                                                            ^

Found on Ubuntu 16.04 LTS with gtk+ 3.18.0.

Stop double assigning to gscreen.  Fixes commit:
    26f4dc50
    Replace deprecated gtk_show_uri() method for help window (!82)
4 jobs for master in 33 minutes and 8 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed centos_build #1522273

00:10:46

passed ubuntu_build #1522274

00:02:58

 
  Test
passed centos_test #1522275

00:15:24

passed ubuntu_test #1522276

00:22:21