hw/audio/virtio-snd: Remove unnecessary "exec/tswap.h" header

hw/audio/virtio-snd: Remove unnecessary "exec/tswap.h" header

We were including the "exec/tswap.h" header to get
target_words_bigendian() declaration, but since commit a276ec8e26
("hw/audio/virtio-snd: Always use little endian audio format")
removed this method call, we don't need this header anymore.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>