config: Include errno.h in mkmakefile.cc

Commit da8884202940 ("config: error out on malformed env/hint lines")
added a reference to EINVAL. In some configurations the bootstrap tools
build fails for lack of errno definitions.

Fixes: da8884202940 ("config: error out on malformed env/hint lines")
Reported by: syzbot+b1a5d112a737d9a2be9b@syzkaller.appspotmail.com