clock_gettime.2: fix markup

The CLOCK_* constants are "defined variable or preprocessor constants"
and so use .Dv.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45106

(cherry picked from commit 2d29d2ecebf8ea19221995b3ea2e3a7ac700bf81)