Fix build on systems with .y.o rules

Fix build on systems with .y.o rules

On systems with system .y.o rules (such as FreeBSD), the system
rule for making error_table.o from error_table.y can bypass
AFS_CCRULE and thus fail to pull in the necessary include paths
for compilation.  Present an explicit dependency on error_table.c
to force that file to be generated, and then our .c.o rule gets
used as desired.

Change-Id: I88a6cf15441e0bacd73888b941d6c268786c5d2a
Reviewed-on: http://gerrit.openafs.org/3145
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>