commit 385a6d5877258cee0cac6151e6359c9206006b01 removed src/lib9p/_post.c
from the code base, but overlooked removing a reference to the
_post.o object file from the src/lib9p/mkfile.
This results in lib9p failing to compile:
* Running on Darwin...
* Compiler version:
Apple clang version 12.0.5 (clang-1205.0.22.11)
* Building mk...
* Building everything (be patient)...
>>> mk: don't know how to make '/Users/sasha/plan9port_fork/lib/lib9p.a(_post.o)' in /Users/sasha/plan9port_fork/src/lib9p
mk: for i in ... : exit status=exit(1)
Remove _post.o from the list of dependent object files from
src/lib9p/mkfile to have lib9p compile.
Fixes: 385a6d587725 ("lib9p: Remove postmountsrv (#505)")