codal_port/modspeech: Allow background processing (#108)

Motivated by the simulator work, allow background processing while
waiting for the speech output to be drained.

This is needed to give the browser an opportunity to schedule the audio.

Author: microbit-matt-hillsdon

src/codal_port/modspeech.c

@@ -115,6 +115,8 @@ STATIC void speech_wait_output_drained(void) {
     #if USE_DEDICATED_AUDIO_CHANNEL
     while (speech_output_read >= 0) {
         mp_handle_pending(true);
+        extern void microbit_hal_background_processing(void);
+        microbit_hal_background_processing();
     }
     uint32_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
     int x = speech_output_read;