kernel: move syscall_handler.h to internal include directory

Move the syscall_handler.h header, used internally only to a dedicated
internal folder that should not be used outside of Zephyr.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/stack.c b/kernel/stack.c
index dc6d1a6..5e00b76 100644
--- a/kernel/stack.c
+++ b/kernel/stack.c
@@ -16,7 +16,7 @@
 #include <wait_q.h>
 #include <zephyr/sys/check.h>
 #include <zephyr/init.h>
-#include <zephyr/syscall_handler.h>
+#include <zephyr/internal/syscall_handler.h>
 #include <kernel_internal.h>
 
 #ifdef CONFIG_OBJ_CORE_STACK