This was still conditional on whether SDL2::SDL2-static exists, but
it's now SDL3 that matters.
Resolves: https://github.com/libsdl-org/SDL/issues/6801
Fixes: 9eb4d1f "cmake: use FindALSA.cmake to find ALSA libraries"
Signed-off-by: Simon McVittie <smcv@collabora.com>