kernel: cmake: fix k_sleep compilation error for single thread

Fix k_sleep compilation error:
[build] ... syscalls/kernel.h:135: undefined reference to `z_impl_k_sleep'
for single thread applications (CONFIG_MULTITHREADING = n).
The shed.c contains source code which must be present also
in single thread applications.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>