sys: arch_interface: include arch/syscall.h if USERSPACE

Due to arch_syscall_invoke are declared static, if there are
no corresponding definition, compiler will complain. It has
been working fine so far because arch/syscall.h is included
indirectly through some other headers (especially through
generated syscall stubs). However, with enough header files
shuffling this becomes an issue. So include arch/syscall.h
in arch_interface.h explicitly.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>