sh: userimask.c needs linux/stat.h

This fix the problem that S_IRUSR and S_IWUSR are not solved in userimask.c.

-----
CC      drivers/usb/host/r8a66597-hcd.o
drivers/sh/intc/userimask.c:57: error: ‘S_IRUSR’ undeclared here (not in a function)
drivers/sh/intc/userimask.c:57: error: ‘S_IWUSR’ undeclared here (not in a function)
CC      drivers/watchdog/shwdt.o
-----

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>