linux-user: Fix wrong type used for argument to rt_sigqueueinfo

linux-user: Fix wrong type used for argument to rt_sigqueueinfo

The third argument to the rt_sigqueueinfo syscall is a pointer to
a siginfo_t, not a pointer to a sigset_t. Fix the error in the
arguments to lock_user(), which meant that we would not have
detected some faults that we should.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>