ejabberd: Replace $(HOST_DIR) to /usr in ERL path

Previously, it was working by luck. Buildroot has fixed its definition
of HOST_DIR and pkg-autotools.mk uses the classical /usr prefix. So,
fix this sed expression to correctly replace $(HOST_DIR) by /usr in ERL
path.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>