Fix t/locking.t under (at least) FreeBSD 9

Trying to `cpanm Path::Tiny` under FreeBSD produces the following error:

    #   Failed test 'Test::FailWarnings should catch no warnings'
    #   at t/locking.t line 20.
    # Warning was 'Argument "LOCK_EX" isn't numeric in flock at t/locking.t line 20.'
    # Tests were run but no plan was declared and done_testing() was not seen.
    t/locking.t ............. skipped: Can't lock tempfiles on this OS/filesystem

Removing the quotes around LOCK_EX fixes the problem.
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