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/timer.c b/kernel/timer.c
index 1dc60db..bd69e35 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -7,7 +7,7 @@
 #include <zephyr/kernel.h>
 
 #include <zephyr/init.h>
-#include <zephyr/syscall_handler.h>
+#include <zephyr/internal/syscall_handler.h>
 #include <stdbool.h>
 #include <zephyr/spinlock.h>
 #include <ksched.h>