sh: plug vsyscall dir in to archclean.

The vsyscall targets are presently not cleaned up, so just handle it in
the archclean rule.
Reported-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/Makefile

@@ -205,6 +205,7 @@ archprepare: maketools
 
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
+	$(Q)$(MAKE) $(clean)=arch/sh/kernel/vsyscall
 
 define archhelp
 	@echo '* zImage 	           - Compressed kernel image'