Mark that it doesn't work with GNAT CE 2020 (#166)

This way our tests depending on this crate should skip this compiler version,
and users know this is an expected failure.

Once SDLAda 2.5.4 is released with the fix a new release for CE 2020 will fix
the issue for good.

index/sd/sdlada/sdlada-2.3.1.toml

@@ -11,6 +11,8 @@ project-files = ["build/gnat/sdlada.gpr"]
 libsdl2 = "^2.0"
 libsdl2_image = "^2.0"
 libsdl2_ttf = "^2.0"
+gnat = "/=2020" # Does not build yet with CE 2020
+
 [[actions."case(os)".linux]]
 type = "post-fetch"
 command = ["make", "-C", "build/gnat", "SDL_PLATFORM=linux", "SDL_MODE=release"]