remove the constness of the two code strings because this does not pass boost_concept...

remove the constness of the two code strings because this does not pass boost_concept_check with gcc-3.3.4-11:

 g++ -/usr/include/g++/bits/boost_concept_check.h: In member function `
   lyx::IconvProcessor& lyx::IconvProcessor::operator=(const
   lyx::IconvProcessor&)':
/usr/include/g++/bits/boost_concept_check.h:203:   instantiated from `void __gnu_cxx::_SGIAssignableConcept<_Tp>::__constraints() [with _Tp = lyx::IconvProcessor]'
/usr/include/g++/bits/stl_map.h:94:   instantiated from `std::map<std::string, lyx::IconvProcessor, std::less<std::string>, std::allocator<std::pair<const std::string, lyx::IconvProcessor> > >'
unicode.C:281:   instantiated from here
/usr/include/g++/bits/boost_concept_check.h:203: error: non-static const member
   `const std::string lyx::IconvProcessor::tocode_', can't use default
   assignment operator
/usr/include/g++/bits/boost_concept_check.h:203: error: non-static const member
   `const std::string lyx::IconvProcessor::fromcode_', can't use default
   assignment operator

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15612 a592a061-630c-0410-9148-cb99ea01b6c8