libpam: Add pam package to DISTRO_FEATURES variable

Fix the libpam compiling warning caused by the missing of pam package
from the DISTRO_FEATURE variable:
WARNING: Building libpam but 'pam' isn't in DISTRO_FEATURES, PAM won't
work correctly

Signed-off-by: Daniel Dragomir <daniel.dragomir@windriver.com>