gettext: remove useless ABOUT-NLS file

For some reason, gettext installs on the target a documentation file
named ABOUT-NLS in /usr/share/gettext, that is clearly not needed for
the proper execution of programs. This commit adds a post-install hook
in gettext.mk to get rid of this file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>