build: separate source and header compile_et rules

Generate source and header files separately to support parallel make
without contortions.

Add a complete list of dependencies for each generated header file
to avoid build errors during parallel make.

Change-Id: I804ff553e08d411a1cfe20a4ef4e57da9d321837
Reviewed-on: http://gerrit.openafs.org/10370
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>