Fix linking error by explicitly instantiating a function template

[Qrox]

Summary

SUMMARY: None

Purpose of change

advanced_inv_pane.cpp uses the function template advanced_inv_area::i_stacked, which is declared in advanced_inv_area.h, but defined in advanced_inv_area.cpp. This somehow causes linking to fail with MinGW-w64 on Windows, even though the function template should have been instantiated in advanced_inv_area.cpp, where it is also used.

Describe the solution

Explicitly instantiate the function template in advanced_inv_pane.cpp.

Testing

Link succeeded after the fix.

[Qrox]

To add to the oddity, apparently the failure only happens with the Release configuration of CMake build.