Set proper sdl2 min bound

[TristanCacqueray]

This change prevents building against sdl2 version 1 which does not have the necessary exposed modules.

[TristanCacqueray]

Not sure how this happened, but in a new environment, cabal somehow picked sdl2 version 1 which was failing with this mysterious error:

src/DearImGui/SDL.hs:44:1: error: [GHC-87110]
    Could not load module `SDL'.
    It is a member of the hidden package `sdl2-2.5.5.0'.
    Perhaps you need to add `sdl2' to the build-depends in your .cabal file.
   |
44 | import SDL
   | ^^^^^^^^^^

This change should prevent that.