vmcore-dmesg/vmcore-dmesg.c: Fix shifting error reported by cppcheck

Running 'cppcheck' static code analyzer (see cppcheck(1))
 on 'vmcore-dmesg/vmcore-dmesg.c' shows the following
shifting error:

$ cppcheck  --enable=all  vmcore-dmesg/vmcore-dmesg.c
Checking vmcore-dmesg/vmcore-dmesg.c ...
[vmcore-dmesg/vmcore-dmesg.c:17]: (error) Shifting signed 32-bit value by 31 bits is undefined behaviour

Fix the same via this patch.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>