[concurrency] Provide missing header file. (#37004)
[concurrency] Provide missing header file. (#37004)

HAVE_PTHREAD_H is supplied in this config file.

The `__has_include` pattern is discouraged, but there is no other way of
conditioning on `pthread_np.h`, which is required for `pthread_main_np`
on OpenBSD. Therefore, use it, but call it out in a comment.
tkremenek/swift,ahoppen/swift,JGiola/swift,ahoppen/swift,xwu/swift,glessard/swift,parkera/swift,xwu/swift,JGiola/swift,ahoppen/swift,xwu/swift,rudkx/swift,hooman/swift,atrick/swift,glessard/swift,hooman/swift,JGiola/swift,rudkx/swift,benlangmuir/swift,atrick/swift,rudkx/swift,rudkx/swift,parkera/swift,ahoppen/swift,apple/swift,atrick/swift,apple/swift,atrick/swift,roambotics/swift,hooman/swift,benlangmuir/swift,parkera/swift,roambotics/swift,apple/swift,roambotics/swift,apple/swift,JGiola/swift,parkera/swift,apple/swift,gregomni/swift,tkremenek/swift,benlangmuir/swift,parkera/swift,benlangmuir/swift,tkremenek/swift,rudkx/swift,tkremenek/swift,JGiola/swift,roambotics/swift,parkera/swift,glessard/swift,gregomni/swift,parkera/swift,benlangmuir/swift,tkremenek/swift,atrick/swift,roambotics/swift,apple/swift,glessard/swift,ahoppen/swift,tkremenek/swift,glessard/swift,xwu/swift,rudkx/swift,atrick/swift,xwu/swift,hooman/swift,parkera/swift,gregomni/swift,xwu/swift,hooman/swift,roambotics/swift,tkremenek/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,xwu/swift,ahoppen/swift,hooman/swift,gregomni/swift,hooman/swift,glessard/swift,gregomni/swift