Fix a warning shown up with clang++: -Wall -Werror -Wextra

[cjatin]

elfio/elfio_modinfo.hpp:62:26: warning: loop variable '[first, second]' creates a copy from type 'std::pair<std::basic_string<char>, std::basic_string<char>> const' [-Wrange-loop-construct]
   62 |         for ( const auto [first, second] : content ) {
      |                          ^
elfio/elfio_modinfo.hpp:62:15: note: use reference type 'std::pair<std::basic_string<char>, std::basic_string<char>> const &' to prevent copying
   62 |         for ( const auto [first, second] : content ) {
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                          &
1 warning generated.

[serge1]

Accepted. Thank you!