ignition: Add partprobe to initrd

The sgdisk tool does not update the kernel partition table in contrast
to other similar tools. Often udev can detect the changes but not always
as experienced when adding a new partition on Flatcar's boot disk.
Instead of implicitly relying on some other component to re-read the
kernel partition table, trigger the re-read with partprobe. This is
proposed in coreos/ignition#1717

dracut/30ignition/module-setup.sh

@@ -37,6 +37,7 @@ install() {
         mkswap \
         nvme \
         sgdisk \
+        partprobe \
         useradd \
         userdel \
         usermod \