security/libu2f-host: Unbreak build with DOCS disabled

libu2f-host does not successfully configure unless gtk-doc is present
and enabled:

===>  Configuring for libu2f-host-1.1.10_1
/bin/sh: /usr/local/bin/gtkdocize: not found
===>  Mk/Uses/autoreconf.mk: Error running gtkdocize
*** Error code 1

So: always enable gtk-doc and build DOCS things just don't install
them.

PR: 264298