WPA: Allow CLOCK_BOOTTIME and CLOCK_MONOTONIC to #define the same

Historically, these have been differnet values, and only one was defined
or they were defined as different values. Now that they are about to be
the same value, add #ifdef to cope.

Sponsored by: Netflix
Reviewed by: olce, val_packett.cool, adrian
Differential Revision: https://reviews.freebsd.org/D45418