sigutil: Include <linux/errno.h>

This file introduced in 2.6.32.47 currently fails to compile:

arch/sparc/kernel/sigutil_64.c: In function 'save_fpu_state':
arch/sparc/kernel/sigutil_64.c:25: error: 'EFAULT' undeclared (first use in this function)

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
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