Fix -Werror=misleading-indentation errors seen with gcc 6

Fix -Werror=misleading-indentation errors seen with gcc 6

This looks like an actual bug which has been lurking here since forever,
fortunately not exposed since hardly anything uses Option::Optional...

libgetopt++/src/OptionSet.cc: In member function 'void OptionSet::doOption(std::__cxx11::string&, const size_type&)':
libgetopt++/src/OptionSet.cc:125:25: error: this 'if' clause does not guard... [-Werror=misleading-indentation]
                         if (!isOption(maybepos))
                         ^~
libgetopt++/src/OptionSet.cc:128:8: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'if'
        argv.erase(argv.begin() + 1);
        ^~~~
libgetopt++/src/OptionSet.cc:159:25: error: this 'if' clause does not guard... [-Werror=misleading-indentation]
                         if (!isOption(maybepos))
                         ^~
libgetopt++/src/OptionSet.cc:161:8: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'if'
        argv.erase(argv.begin() + 1);
        ^~~~