Replace 'struct siginfo' with 'siginfo_t'.

Replace 'struct siginfo' with 'siginfo_t'.

glibc 2.16 will remove the undocumented definition of 'struct siginfo'
from <bits/siginfo.h>.

This change is already present in glibc 2.15.90, so qemu compilation
of certain targets (eg. cris-user) breaks.

This struct was always typedef'd to be the same as 'siginfo_t' which
is what POSIX documents, so use that instead.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>