add XML::free and XML::xmlChildren to NAMESPACE

Dear Thomas,

we importing your package in `MSnbase` and get the following errors:

    Error in flatten(mzID(filename, verbose = verbose)) : 
      error in evaluating the argument 'object' in selecting a method for function 'flatten': Error in mzID(filename, verbose = verbose) : 
      could not find function "xmlChildren"

    Error in flatten(mzID(filename, verbose = verbose)) : 
      error in evaluating the argument 'object' in selecting a method for function 'flatten': Error in mzID(filename, verbose = verbose) : 
      could not find function "free"

It seems there are some missing `importFrom` commands in your NAMESPACE file.

BTW: Why do you depend on the `XML` package rather than import it?

Best wishes,

Sebastian