gtk: don't leak the GtkBorder with VTE 0.36

When gtk_widget_style_get() is used to get the "inner-border" style
property, it returns a copy of the GtkBorder which must be freed by
the caller.

This patch also fixes a warning about the unused 'padding' structure
with VTE 0.36.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1463127654-5171-1-git-send-email-berto@igalia.com
Cc: Cole Robinson <crobinso@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>

[ kraxel: adapted to changes in ui patch queue ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
