runtime/cgo: fix building on musl
sys/unistd.h only exists in glibc and not in musl so use the standard
location. This is a regression from CL 210639
Change-Id: Idd4c75510d9829316b44300c36c34df6d667cc05
GitHub-Last-Rev:
0fa4162f1c7c460bda7585300285f47d1781985d
GitHub-Pull-Request: golang/go#43038
Reviewed-on: https://go-review.googlesource.com/c/go/+/275732
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Andrew G. Morgan <agm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>