linux-pam: unavailable for static builds

Fixes:
http://autobuild.buildroot.net/results/3bb/3bb8d4662995a11e8e6cc5fdaaba9e25d4c0e0ec/



[Thomas: add more details about the non availability for static builds
as a comment in the Config.in file.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>