Make the XFS support optional, not required.

With the xfs_growfs command required, the provider was failing
suitability tests and unavailable if the xfs_growfs command was not
installed:

debug: Puppet::Type::Logical_volume::ProviderLvm: file xfs_growfs does
not exist
err: Could not find a suitable provider for logical_volume
Merge pull request #42 from smithj4/xfs-optional

Make the XFS support optional, not required.