Explicitly qualify specializations of elfutils::to_string

Explicitly qualify specializations of elfutils::to_string

I attempted to build the "dwarflint" branch with gcc 7.3.1 , but ran
into various problems of the form:

c++/known.cc:220:1: error: explicit specialization of ‘template<class type> std::__cxx11::string elfutils::to_string(const type&)’ outside its namespace must use a nested-name-specifier [-fpermissive]
 to_string<dwarf::dwarf_enum> (const dwarf::dwarf_enum &value)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The fix appears to be trivial.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>