issue with cast (dictionarySearch) with SGI compiler

• as reported on the cfd-online forum,

    db/dictionary/dictionarySearch.C(327): error: invalid type conversion:
    "Foam::dictionary::const_searcher" to "const Foam::dictionary::searcher &"
    return static_cast<const searcher&>(finder);

The conversion should be OK (we use it for switching between const and non-const forms of the searcher), but obviously not the case for SGI.