package/dovecot: add linux-pam optional dependency

Fixes:
 - http://autobuild.buildroot.org/results/bba0d54cab164d77caf7161596b22602875a7a85



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>