runtime/cgo: fix building on musl
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew G. Morgan <b5c5c384dbc594e131e4ee70f9c28e46b1963602@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

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