logd: make drop_privs work if neither klogd or auditd are used

Fix a bug that was causing cap_set_flag() fail and logd to exit.

Bug: 159588327
Test: caps are set correctly and logd functions with both, one of, or
      none of klogd and auditd enabled.
Change-Id: Ia51f078ad848535ce1ac29edd8a56a2b686a12cc