audio: Disable NEON sample conversion until test failures are fixed
We need to do this early in the file, so that it will be taken into
account when deciding whether to define NEED_SCALAR_CONVERTER_FALLBACKS
and therefore provide a non-SIMD fallback.
Mitigates: https://github.com/libsdl-org/SDL/issues/8352
Signed-off-by: Simon McVittie <smcv@collabora.com>