internal/poll: add FD.Fsync on aix

Follow-up for CL 138717. This fixes the build of the os package on
aix.

Change-Id: I879b9360e71837ab622ae3a7b6144782cf5a9ce7
Reviewed-on: https://go-review.googlesource.com/c/141797
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/poll/fd_fsync_posix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris windows
+// +build aix dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris windows
 
 package poll