Default to --with-blkid in RPM packaging

Support for libblkid has been in the ZoL for years but has been
disabled by default in the packaging.  When libblkid is disabled
the 'zpool create' command will not warn you if you're going to
overwrite an existing filesystem.  For this reason the default
behavior has been changed to require libblkid.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2448