posix: semaphores: use a default minimal heap-add for semaphores

The implementation of POSIX_SEMAPHORES historically used heap allocation
and has not yet been transitioned to a pool allocator.

However, since 590258b3, the default heap-add with CONFIG_POSIX_API
has been reduced from 1 kiB which causes tests/posix/semaphores to fail
due to NULL being returned from a call to k_calloc().

Create a minimal heap-add for the POSIX_SEMAPHORES Option Group.

This can be removed at a future date if semaphores are changed to use
a pooled allocator and fixed-size name, rather than heap allocation.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>