New unused variables warning in nx_start()

[Nathan Hartman]

In function nx_start() (sched/init/nx_start.c), if none of
MM_KERNEL_USRHEAP_INIT, CONFIG_MM_KERNEL_HEAP, or CONFIG_MM_PGALLOC
are defined, the compiler warns about unused variables heap_start and
heap_size:

init/nx_start.c: In function 'nx_start':
init/nx_start.c:552:14: warning: unused variable 'heap_size' [-Wunused-variable]
       size_t heap_size;
              ^~~~~~~~~
init/nx_start.c:551:17: warning: unused variable 'heap_start'
[-Wunused-variable]
       FAR void *heap_start;
                 ^~~~~~~~~~

I wonder if there is something wrong with my configuration (should one
of these always be defined?) or if it's valid to have none of them
defined and I should make a change that eliminates the warning?

Thanks,
Nathan