[3.x] Fix gcc warning packed scene

[lawnjelly]

Backport of #111771

Notes

• We don't have clipper2 in 3.x, so only require this part.
• This doesn't fail CI in 3.x, but might be useful for newer versions of GCC.
• I'm not completely convinced this is worth changing, but included as an option.

[akien-mga]

So it seems it's not failing on CI currently because we don't build with mingw-gcc 15 on Windows, unlike on master.

I did a quick test with mingw-gcc 15 in a Fedora 43 container and found a number of other warnings (but not this one, might depend on optimization level), so they should probably be tackled in a dedicated PR and not just cherry-picking this single change. It's not a blocker for 3.6.2 anyway since it's not breaking our current CI, so I think we can skip this one.

Build log from scons p=windows warnings=extra on Fedora 43 with mingw-gcc 15.2.1:

godot-3.x-f43-mingw-gcc-15.2.1.log