package/unscd: disable on riscv32

unscd unconditionally uses __NR_clock_gettime which will raise the
following build failure on riscv32:

nscd-0.54.c:339:14: error: '__NR_clock_gettime' undeclared (first use in this function); did you mean 'clock_gettime'?
  339 |  if (syscall(__NR_clock_gettime, CLOCK_MONOTONIC, &ts))
      |              ^~~~~~~~~~~~~~~~~~
      |              clock_gettime

Fixes:
 - http://autobuild.buildroot.org/results/eb77b18f268d8e59c407f757662117a33d3f9ee3



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>