lib9p: Fix compile error

[alexkapshuk]

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)")

[bhuntsman]

My mistake, I had that locally but missed it in my commit. I pinged Dan Cross asking him to merge this commit. My apologies!

[alexkapshuk]

Thanks.