syscalls: pull in arch/cpu.h

Need the implementation of _arch_syscall_invokeX() functions
to avoid build problems in some cases.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/include/syscall.h b/include/syscall.h
index d9721c9..281efa6 100644
--- a/include/syscall.h
+++ b/include/syscall.h
@@ -12,6 +12,7 @@
 #include <zephyr/types.h>
 #include <syscall_list.h>
 #include <syscall_macros.h>
+#include <arch/cpu.h>
 
 #ifdef __cplusplus
 extern "C" {