python-lxml: allow build as host package

python-lxml: allow build as host package

While currently there is no in-tree Buildroot package which depends on
host-python-lxml, we (DATACOM) have some proprietary modules that use it
in their test scripts.

We tested python-lxml as host package and confirmed that it builds and
works correctly. Someone else might require it, so we are proposing its
inclusion.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
[Thomas: add Config.in.host entry.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>