runtime: include pthread.h in defs_openbsd.go

This is required now that defs_openbsd.go has pthread related references.

Updates #36435

Change-Id: I73cdf23eef6aceea6f9b37b7702bdb3b560aa120
Reviewed-on: https://go-review.googlesource.com/c/go/+/334877


Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>