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>

kernel/CMakeLists.txt

@@ -59,6 +59,7 @@ list(APPEND kernel_files
   mem_slab.c
   thread.c
   version.c
+  sched.c
   )
 
 if(CONFIG_MULTITHREADING)
@@ -72,7 +73,6 @@ list(APPEND kernel_files
   stack.c
   system_work_q.c
   work.c
-  sched.c
   condvar.c
   )