Fix build with separate objdirs

The conversion of pam/ to use libtool introduced references to
file names with LT_CCRULE that are not subject to make's vpath
searching.

Sprinkle ${srcdir} accordingly to fix the build.

Change-Id: Ia500fe2a57813fdd93ca1767e243fd947d6b8e1e
Reviewed-on: http://gerrit.openafs.org/9677
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>