sdl3: Fix SDL_GetPrefPath() SDL_free()

[RobLoach]

We should likely make sure SDL_GetPrefPath() call uses SDL_free(). The PhysFS allocator may be different than SDL's, so this ensures it's cleared by the same allocator across both uses.

• I confirm that I am the author of this code and release it to the PhysicsFS project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

[icculus]

I beat you to this one, in bf429aa. :)