As gdk_keymap_get_default() was deprecated in 3.22

Warnings fixed:
 | vncdisplaykeymap.c: In function ‘vnc_display_keyval_set_entries’:
 | vncdisplaykeymap.c:263:4: warning: ‘gdk_keymap_get_default’ is
 | deprecated: Use 'gdk_keymap_get_for_display' instead
 |
 |     gdk_keymap_get_entries_for_keyval(gdk_keymap_get_default(),
 |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 |
 | spice-gtk-session.c: In function ‘spice_gtk_session_init’:
 | spice-gtk-session.c:181:5: warning: ‘gdk_keymap_get_default’ is
 | deprecated: Use 'gdk_keymap_get_for_display' instead
 |
 |     GdkKeymap *keymap = gdk_keymap_get_default();
 |     ^~~~~~~~~
 |
 | spice-widget.c: In function ‘get_scancode_from_keyval’:
 | spice-widget.c:1731:5: warning: ‘gdk_keymap_get_default’ is
 | deprecated: Use 'gdk_keymap_get_for_display' instead
 |
 |      if (gdk_keymap_get_entries_for_keyval(gdk_keymap_get_default(),
 |      ^~

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>