gettext-full: fix m4 path after gettextize update

gettext-full: fix m4 path after gettextize update

Recent changes to gettextize altered the default path for .m4 files from
$datadir/aclocal to $datadir/gettext/m4 [0]. This caused build issues when
compiling gettext-full in OpenWrt.

This patch, originally provided by @nxhack [1], updates the OpenWrt
Makefile accordingly to ensure compatibility with the new path.

[0] https://gitweb.git.savannah.gnu.org/gitweb/?p=gettext.git;a=commit;h=fa98427c774aad9dade7702becc2c3eef5a8434d
[1] https://github.com/openwrt/openwrt/commit/da541f7acd62bc33e7b0a891cf65a39d4bfe0b96#commitcomment-163048847

Fixes: da541f7acd62 ("gettext-full: update to 0.24.1")
Link: https://github.com/openwrt/openwrt/pull/19614
Signed-off-by: Nick Hainke <vincent@systemli.org>