syscall: define ESPIPE on Plan 9

TestSeekError has been added in CL 41311. This
test doesn't build on Plan 9 because syscall.ESPIPE
is not defined on Plan 9.

This change defines syscall.ESPIPE on Plan 9.

Fixes #20078.

Change-Id: I3d9e95b00e0c4e43312eada6441d80961ae6bd67
Reviewed-on: https://go-review.googlesource.com/41471
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/syscall/errors_plan9.go

@@ -45,6 +45,7 @@ var (
 	// what package os and others expect.
 	EACCES       = NewError("access permission denied")
 	EAFNOSUPPORT = NewError("address family not supported by protocol")
+	ESPIPE       = NewError("illegal seek")
 )
 
 // Notes