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.

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>
(cherry picked from commit 4e6b7ab904d38d38da1b80a7342bd815668a8c09)

Change-Id: Ic1474aff86e49d8df0ba151ed0e53932aa96d1ed
Reviewed-on: http://gerrit.openafs.org/11239
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>