missing isc_mutex_destroy

timer_test fails on the Mac.

[==========] Running 3 test(s).
[ RUN      ] once_idle
[  ERROR   ] --- Blocks allocated...
timer_test.c:94: note: block 0x10341cc00 allocated here
ERROR: once_idle leaked 1 block(s)

[  FAILED  ] once_idle
[ RUN      ] reset
[  ERROR   ] --- Blocks allocated...
timer_test.c:94: note: block 0x10341cb40 allocated here
ERROR: reset leaked 1 block(s)

[  FAILED  ] reset
[ RUN      ] purge
[       OK ] purge
[==========] 3 test(s) run.
[  PASSED  ] 1 test(s).
[  FAILED  ] 2 test(s), listed below:
[  FAILED  ] once_idle
[  FAILED  ] reset

 2 FAILED TEST(S)