whois: fix build with NLS disabled

whois: fix build with NLS disabled

whois build uses the xgettext utility to build translation files.
This utility may not be available when NLS is disabled. Omit the pos and
install-pos Makefile targets from the default and the install targets
when NLS is disabled, respectively.

Fixes:
http://autobuild.buildroot.net/results/862/862d8165a59711c970c65d90009a527315a1a6b4/
http://autobuild.buildroot.net/results/b62/b62ec9d6f350e409731d47ffb585344c516944e8/
http://autobuild.buildroot.net/results/be2/be28d06ae004a46c87133d6f07709c6f909288d8/

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>