Error when disabling timing#44

I defined the `MUNIT_DISABLE_TIMING` macro and i get compile errors. ``` munit.c:904:10: error: variable ‘wc’ has initializer but incomplete type struct PsnipClockTimespec wc = { 0, }; ^~~~~~~~~~~~~~~~~~ munit.c:903:29: error: storage size of ‘wc’ isn’t known struct PsnipClockTimespec wc = { 0, }; munit.c:906:24: error: ‘PSNIP_CLOCK_TYPE_WALL’ undeclared (first use in this function) psnip_clock_get_time(PSNIP_CLOCK_TYPE_WALL, &wc); ```