[MiNT] writev() on MiNT

[David Brownlee <abs@absd.org>]

	Trying to build some apps on MiNT, and noticed they can't seem
	to find writev(). This is on 1.16.3a.

	The FreeMiNT syscalls.master seems to indicate that writev()
	should be available since 1.16 - am I missing anything?

	Thanks