[release-branch.go1.16] runtime/cgo: add cast in C code to avoid C compiler warning

For #44340
Fixes #44346

Change-Id: Id80dd1f44a988b653933732afcc8e49a826affc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/293209


Reviewed-by: Andrew G. Morgan <agm@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 07ef3135)
Reviewed-on: https://go-review.googlesource.com/c/go/+/293411
TryBot-Result: Go Bot <gobot@golang.org>