Fix compilation: use reference in loop, not copy

[anothersimulacrum]

Summary

SUMMARY: None

Purpose of change

src/advanced_inv.cpp:823:38: error: loop variable 'elem' of type 'const advanced_inv_listitem' creates a copy from type 'const advanced_inv_listitem' [-Werror,-Wrange-loop-construct]
    for( const advanced_inv_listitem elem : spane.items ) {
                                     ^
src/advanced_inv.cpp:823:10: note: use reference type 'const advanced_inv_listitem &' to prevent copying
    for( const advanced_inv_listitem elem : spane.items ) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                     &
1 error generated.

Describe the solution

Make a reference

Testing

It compiles.