Add support for RTP with GSM-HR payload according to RFC5993

This is incompatible with the ETSI TS 101 318 format!
diff --git a/src/formats.c b/src/formats.c
index 655a7be..60182fe 100644
--- a/src/formats.c
+++ b/src/formats.c
@@ -38,6 +38,7 @@
 extern const struct format_desc fmt_rtp_amr;
 extern const struct format_desc fmt_rtp_efr;
 extern const struct format_desc fmt_rtp_hr_etsi;
+extern const struct format_desc fmt_rtp_hr_ietf;
 
 static const struct format_desc *supported_formats[_FMT_MAX] = {
 	[FMT_INVALID]		= NULL,
@@ -56,6 +57,7 @@
 	[FMT_RTP_AMR]		= &fmt_rtp_amr,
 	[FMT_RTP_EFR]		= &fmt_rtp_efr,
 	[FMT_RTP_HR_ETSI]	= &fmt_rtp_hr_etsi,
+	[FMT_RTP_HR_IETF]	= &fmt_rtp_hr_ietf,
 };