drivers: power_supply_sysfs.c needs stat.h

It was actually getting this before by a tangled mess of implict
includes that is going to be cleaned up.  Fix it now, so we don't
get this after the cleanup.

power_supply_sysfs.c: In function ‘power_supply_attr_is_visible’:
power_supply_sysfs.c:184: error: ‘S_IRUSR’ undeclared (first use in this function)
power_supply_sysfs.c:184: error: (Each undeclared identifier is reported only once
power_supply_sysfs.c:184: error: for each function it appears in.)
power_supply_sysfs.c:184: error: ‘S_IRGRP’ undeclared (first use in this function)
power_supply_sysfs.c:184: error: ‘S_IROTH’ undeclared (first use in this function)
power_supply_sysfs.c:196: error: ‘S_IWUSR’ undeclared (first use in this function)
make[3]: *** [drivers/power/power_supply_sysfs.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>