budb: Only have one build rule for budb_errs.c

budb: Only have one build rule for budb_errs.c

budb had two build rules that could produce budb_errs.c, but only
one of them also produced budb_client.h. This led to problems with
parallel makes, as depending on which rule fired first, budb_client.h
might, or might not, exist.

Rework all of this so that it's cleaner. Instead of producing two
copies of the error table, just make budb_client.h a static file,
and include the dynamically generated budb_errs.h from it. This
reduces code duplication, and means that we have to run compile_et
one less time.

Change-Id: Ib9b982454cc3f1ef3bdeb629c64dbb9901397fdb
Reviewed-on: http://gerrit.openafs.org/4027
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>