package/gettext: host variant needs host-libxml2

Of host-gettext, we only build the tools. however some of those tools
have a --color option, which needs libxml2.

If the libxml2 development files are missing on the host, a bundled
subset of libxml2 is used, and all seems OK. But if they ar epresent,
then the host libxml2 is used. On some distros, lbxml2 is linked with
liblzma. And if we happend to _then_ build our host-liblzma, we may have
a conflict later when trying to run any of the host-gettext tools.

Always build host-gettext with our own host-libxml2 which is not linked
with liblzma.

Fixes https://bugs.busybox.net/show_bug.cgi?id=7886



Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
[nicolas: added extended commit message]
Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
[yann.morin.1998@free.fr: rebase; add comment; enhance commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>