agent: Fix calibrate_get_time use of clock_gettime.

* agent/protect.c (USE_CLOCK_GETTIME): New macro.
(calibrate_get_time): Only use clock_gettime if USE_CLOCK_GETTIME.

• GnuPG-bug-id: T5623
• Fixes-commit: 380bce13d94ff03c96e39ac1d834f382c5c730a1
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>