package: fix reference to host glib-genmarshal

Reported by Sven Neumann <s.neumann@raumfeld.com>

Use LIBGLIB2_HOST_BINARY instead of the non-existing $(HOST_GLIB)
variable to refer to the host version of glib-genmarshal.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>