libgail: mark as deprecated

Mark libgail as deprecated now that midori has been updated to use
webkitgtk24 and the only package that uses it (webkit) has been
deprecated.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>