python-jinja2: allow build as host-package

Jinja2 is a general purpose templating language for python and can be
required during build time for python scripts that generate code.

While currently there are no in-tree Buildroot packages which depend on
host-python-jinja2, I'm currently having to deal with a proprietary
software build system that requires it.

I have tested that it builds and works correctly as a host python
package for a Buildroot host-python and someone else might require it,
so I'm proposing its inclusion.

python-jinja2 for target builds fine without python-markupsafe as a
build dependency, but when building host-python-jinja2 without
host-python-markupsafe installed, the python package manager tries to
download and install it.
To avoid that and install host-python-markupsafe properly through
Buildroot, the host build dependency is needed. Host support for
python-markupsafe was submitted in a previous patch from this patchset.

Signed-off-by: Erico Nunes <erico.nunes@datacom.ind.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>