shared-mime-info: run update-mime-database with correct env variables

When installing shared-mime-info on the target, we runn
update-mime-database, which has been built previously by building and
installing shared-mime-info for the host. However, for
update-mime-database (host version) to run properly, we must pass the
appropriate LD_LIBRARY_PATH variable.

We re-use the existing $(HOST_MAKE_ENV) variable to pass all the
appropriate variables.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>