Fix macos builds from using ungated std::filesystem calls in cata_imgui

[akrieger]

Summary

None

Purpose of change

Fixes

/Applications/Xcode_14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/filesystem:909:24: note: 'path' has been explicitly marked unavailable here
class _LIBCPP_TYPE_VIS path {
                       ^
Error: src/cata_imgui.cpp:275:37: error: 'exists' is unavailable: introduced in macOS 10.15
            return std::filesystem::exists( path );

from #75937

Describe the solution

Use the wrappers we have.

Describe alternatives you've considered

Testing

make RELEASE=1 TILES=1 LOCALIZE=0 SOUND=1 -j10 CCACHE=1 obj/tiles/cata_imgui.o locally.

Additional context