devel/elfio: Fix on GCC architectures

[pkubaj]

Summary

The port requires C++11-compatible compiler:
../../elfio/elfio_strings.hpp:95: error: expected nested-name-specifier before 'string_section_accessor'
../../elfio/elfio_strings.hpp:95: error: 'string_section_accessor' has not been declared
../../elfio/elfio_strings.hpp:95: error: expected `;' before '=' token
../../elfio/elfio_strings.hpp:95: error: expected unqualified-id before '=' token
../../elfio/elfio_strings.hpp:96: error: expected nested-name-specifier before 'const_string_section_accessor'
../../elfio/elfio_strings.hpp:96: error: 'const_string_section_accessor' has not been declared
../../elfio/elfio_strings.hpp:96: error: expected `;' before '=' token
../../elfio/elfio_strings.hpp:96: error: expected unqualified-id before '=' token

Approved by maintainer in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237312

Test Plan

Test build on architectures with GCC in base.