Fail to compile because of usage of undeclared CLOCK_MONOTONIC

../src/gxr-time.c: In function 'gxr_time_age_secs_to_monotonic_msecs':
../src/gxr-time.c:47:7: warning: implicit declaration of function 'clock_gettime' [-Wimplicit-function-declaration]
   47 |   if (clock_gettime (CLOCK_MONOTONIC, &mono_time) != 0)
      |       ^~~~~~~~~~~~~
../src/gxr-time.c:47:22: error: 'CLOCK_MONOTONIC' undeclared (first use in this function)
   47 |   if (clock_gettime (CLOCK_MONOTONIC, &mono_time) != 0)
      |                      ^~~~~~~~~~~~~~~

Not sure if relevant but this happens on a Musl system.