posix: spinlock: additional kconfig for k_spinlock being size 0

The `struct k_spinlock` size is zero bytes under certain
circumstances. This is a bit of a problem, because it breaks a
number of assumptions about things in C.

That should be fixed when #59922 is addressed.

This change is just a hotfix to correct the specific condition
where we will need workarounds in other source files.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>