Fix clang (with its own libc++) build after 9865de7 (#661)

Fix clang (with its own libc++) build after 9865de7 (#661)

Compilers that check allocator/container value_type matching detected a
mismatch because multimap::value_type is _not_ std::pair<key,T> but
std::pair<const key,T>!

Symptoms (when compiling files that include src/PingData.h):

     Allocator::value_type must be same type as value_type