glibc: install the obsolete libnsl

glibc: install the obsolete libnsl

glibc 2.27 stopped installing libnsl by default. Restore libnsl install
to fix packages, like exim, that use that library.

Fixes (exim):
http://autobuild.buildroot.net/results/a19/a19df43be7d27cf8815e1257122d25aa8285d75b/
http://autobuild.buildroot.net/results/8ec/8ecfc42a81efec8485784f945e231eb40a087b5b/
http://autobuild.buildroot.net/results/054/054eb702a9b8c66454970333ef45a0afccb7cc80/

Cc: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>