umtxvar.h: Add missing include

Necessary to have the definition of 'struct timespec'.

No functional change.

Approved by: markj (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit bcaa0b4c2bab2866d5ee11c9eb8900a70bdb21e1)

Approved by: emaste (mentor)