syscall: clean up TestStdcallAndCDeclCallbacks to have no warnings

Fixes #9188

Change-Id: Ifbf5d9fa78a4f4ceb7f92d42494fe37fa7878c45
Reviewed-on: https://go-review.googlesource.com/1930
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>