Add "rtp-efr" format support for RTP payload according to RFC3551 4.5.9

The RTP EFR payload is a bit like the FR payload: one nibble magic
marker, then followed by the actual codec bits.  So we need to
add/remove that magic marker and shift the remainder by one nibble.
diff --git a/src/formats.c b/src/formats.c
index 0d4c4a0..6b2ee91 100644
--- a/src/formats.c
+++ b/src/formats.c
@@ -36,6 +36,7 @@
 extern const struct format_desc fmt_ti_efr;
 extern const struct format_desc fmt_amr_opencore;
 extern const struct format_desc fmt_rtp_amr;
+extern const struct format_desc fmt_rtp_efr;
 
 static const struct format_desc *supported_formats[_FMT_MAX] = {
 	[FMT_INVALID]		= NULL,
@@ -52,6 +53,7 @@
 	[FMT_TI_EFR]		= &fmt_ti_efr,
 	[FMT_AMR_OPENCORE]	= &fmt_amr_opencore,
 	[FMT_RTP_AMR]		= &fmt_rtp_amr,
+	[FMT_RTP_EFR]		= &fmt_rtp_efr,
 };