Properly link zpool command to libblkid

31fc19399e597e3391f19f1392ab120f1de0d5f2 incorrectly removed $(LIBBLKID)
from cmd/zpool/Makefile.am. This meant that the toolchain was not given
-lblkid, which resulted in the following build failure on Ubuntu 13.10:

/usr/bin/ld: zpool_vdev.o: undefined reference to symbol
'blkid_put_cache@@BLKID_1.0'
/lib/x86_64-linux-gnu/libblkid.so.1: error adding symbols: DSO missing
from command line
collect2: error: ld returned 1 exit status

That commit reworked various Makefile.am to follow best practices, so we
reintroduce $(LIBBLKID) in a manner consistent with that, rather than
explicitly reverting the change.

Reproduction of this issue was done on a Gentoo Linux system by
executing the following commands:

zfs create -o mountpoint=/mnt/ubuntu-13.10 rpool/ROOT/ubuntu-13.10
debootstrap --variant=buildd --arch amd64 saucy /mnt/ubuntu-13.10 http://archive.ubuntu.com/ubuntu/
mount -o bind /dev /mnt/ubuntu-13.10/dev/
mount -o bind /proc/ /mnt/ubuntu-13.10/proc/
mount -o bind /sys/ /mnt/ubuntu-13.10/sys/
cp /etc/resolv.conf /mnt/ubuntu-13.10/etc/
(cd /mnt/ubuntu-13.10/root/ && git clone git://github.com/ryao/zfs.git)
chroot /mnt/ubuntu-13.10/
apt-get install autoconf libtool zlib1g-dev uuid-dev libblkid-dev
\#apt-get install git alien fakeroot vim
cd /root/zfs
./autogen.sh
./configure --with-config=user --prefix=/usr
make

That will create a Ubuntu 13.10 chroot, fetch the sources and build
test. At this point, cmd/zpool/Makefile.am was modified and the
following commands were run to verify that the build issue was resolved:

git clean -xdf
./autogen.sh
./configure --with-config=user --prefix=/usr
make

Afterward, the patch was again tested to verify that it did not cause a
regression in the absence of libblkid:

apt-get remove libblkid-dev
git clean -xdf
./autogen.sh
./configure --with-config=user --prefix=/usr
make

Additionally, the commands themselves were tested against my live system
from within the chroot to ensure basic functionality. My live system had
corresponding kernel modules already installed and basic commands such
as `zpool list` and `zfs list` worked without incident. Lastly, this
patch was also build tested on Gentoo Linux, where it caused no
problems.

These steps can be used to reproduce these on any modern Linux system
that has debootstrap installed. On Gentoo, installing debootstrap can be
done with `emerge dev-util/debootstrap`. The current ZFSOnLinux HEAD
revision as of writing is fd23720ae14dca926800ae70e6c8f4b4f82efc08. Once
this is fixed in HEAD, either that revision or another before this fix
and after 31fc19399e597e3391f19f1392ab120f1de0d5f2 will be needed to
reproduce this issue.

Signed-off-by: Richard Yao <ryao@gentoo.org>