package/exim: fix build error with external arc toolchain due to missing libnsl
Fixes
http://autobuild.buildroot.net/results/331/
331ed781b422448205fb9e7c9730ec0c438d6306/
http://autobuild.buildroot.net/results/402/
402d64965ac7ac6e1d4e1990080394958802fe8c/
http://autobuild.buildroot.net/results/960/
9605bac2972d3e3d3fb91947ae6921e89210247b/
http://autobuild.buildroot.net/results/6b6/
6b61ea80a3a6dcead233c4b408eba8b8d647e841/
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>