package/libnss: also install a pkg-config file

Some packages use pkg-config to check for libnss.

Add our own .pc file (vampirised from Debian).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>