package/elixir: fix host-erlang dependency

There is no target elixir package, so setting a value to
ELIXIR_DEPENDENCIES has no effect, HOST_ELIXIR_DEPENDENCIES must be
used instead.

Fixes:

  http://autobuild.buildroot.net/results/a3a37eb724ca5689f8e83c9b2af04d07afa80315/



Signed-off-by: Frank Vanbever <frank.vanbever@essensium.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>