package/gettext-tiny: fix prefix in autopoint

gettext-tiny also installs a fake autopoint, but it still needs to be
pointed at the m4 macros, which the template autopoint expects to be in
@datadir@, where datadir is the traditional autotools datadir, derived
from datarootdir, itself derived from prefix.

So, pass prefix so all the locations are properly pointing to HOST_DIR,
instead of the default /usr/local.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>