runtime/cgo: add cast in C code to avoid C compiler warning
runtime/cgo: add cast in C code to avoid C compiler warning

Fixes #44340

Change-Id: Id80dd1f44a988b653933732afcc8e49a826affc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/293209
Reviewed-by: Andrew G. Morgan <b5c5c384dbc594e131e4ee70f9c28e46b1963602@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go