Remove const qualifier from setter

[Smertig]

This is just a typo fix. Fixes #44

clang-10 refuses to compile library:

.../libelfin/elf/data.hh:558:22: error: cannot assign to non-static data member within const member function 'set_binding'
                info = (info & 0xF) | ((unsigned char)v << 4);
                ~~~~ ^
.../libelfin/elf/data.hh:556:14: note: member function 'elf::Sym<elf::Elf64, Order>::set_binding' is declared const here
        void set_binding(stb v) const
        ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

[emeryberger]

Fixed with https://github.com/aclements/libelfin/pull/79.