The prodosemu port includes the sgtty header file, but doesn't do
anything with it. This is bad habit, especially because the sgtty.h is a
deprecated header file, which describes an interface that's only present
when COMPAT_43TTY is present in the kernel.

There's no real harm in this case, but say, we want to remove this
header file, it will cause compilation to fail.

PR: 119101
Submitted by: Ed Schouten <ed@fxq.nl>