tests: condvar_api: fix empty loop error for XCC

When building with XCC, the for loop to call k_therad_access_grant()
is an empty loop because k_thread_access_grant() does nothing
if no building for userspace. XCC does not like this and emits
error:

  main.s: Assembler messages:
  main.s:1951: Error: invalid empty loop

So add #ifdef to only enable the loop if userspace is enabled.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>