misc/cgo: replace os.SEEK_SET with io.SeekStart

misc/cgo: replace os.SEEK_SET with io.SeekStart

Since os.SEEK_SET was deprecated, use io.SeekStart instead.

Change-Id: I11ae496b071ab35412403ff73e52f3da73d5b120
Reviewed-on: https://go-review.googlesource.com/c/go/+/431096
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>