kernel: move thread_stack.h to include/kernel

Move this internal header file into kernel/ alongside other thread
related headers.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/include/sys/thread_stack.h b/include/kernel/thread_stack.h
similarity index 100%
rename from include/sys/thread_stack.h
rename to include/kernel/thread_stack.h
diff --git a/include/kernel_includes.h b/include/kernel_includes.h
index 1cc0167c..179ca6a 100644
--- a/include/kernel_includes.h
+++ b/include/kernel_includes.h
@@ -36,7 +36,7 @@
 #include <spinlock.h>
 #include <fatal.h>
 #include <irq.h>
-#include <sys/thread_stack.h>
+#include <kernel/thread_stack.h>
 #include <app_memory/mem_domain.h>
 #include <sys/kobject.h>
 #include <kernel/thread.h>