shairport-sync: Enable pipe audio backend support

shairport-sync: Enable pipe audio backend support

UCI's shairport-sync's pipe conf block is defined but has no effect without shairport-sync's pipe support

Rationale for pipe support:
- On SBC with no sound card available (built-in or external)
  and without ALSA plugins support on OpenwRT (no package)
  to reroute a stream from shairport-sync's ALSA default output
  to a PulseAudio controlled Bluetooth sink , then pipe support
  is required to enable the following flow:
  - shairport-sync -> fifo pipe -> custom_binary_to_send_stream_to_pulse_audio_BT_sink
- small bump in package size: diff: +746 bytes
  - shairport-sync-openssl_4.3.2-0_aarch64_generic.ipk without --pipe: 110806 bytes
  - shairport-sync-openssl_4.3.2-0_aarch64_generic.ipk with --pipe: 111552 bytes

Signed-off-by: David Andreoletti <david@andreoletti.net>