libobs: Fix audio keyframe issue

Audio packets are always considered keyframe packets, so if they weren't
already, mark them as keyframe packets.

libobs/obs-encoder.c

@@ -923,6 +923,11 @@ void send_off_encoder_packet(obs_encoder_t *encoder, bool success,
 			encoder->first_received = true;
 		}
 
+		/* if packet is audio, mark as keyframe if it isn't already */
+		if (pkt->type == OBS_ENCODER_AUDIO) {
+			pkt->keyframe = true;
+		}
+
 		/* we use system time here to ensure sync with other encoders,
 		 * you do not want to use relative timestamps here */
 		pkt->dts_usec = encoder->start_ts / 1000 +