libafs: separate source and header compile_et rules

libafs: separate source and header compile_et rules

Use the new compile_et -emit flag to generate source and header
files separately to support parallel make.

Export afs_trace.h since it is required to build libafs.  Before the
compile_et -emit flag was available, The afs_trace.h file was
generated as a side-effect of creating afszcm.cat.

Change-Id: I4e93691dda34ddc8600d6a818503e0c9e75e618a
Reviewed-on: http://gerrit.openafs.org/10729
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>