gsm_08_08: define GSM0808_SCT_EXT (separately)

As per 3GPP TS 48.008, section 3.2.2.103, the "Codec Type" field may
contain either a certain 3GPP Speech Codec Type directly (4 bit value),
or the so called "Codec Extension" = 0xFh, in which case the real Codec
Type follows in the next octet as "Extended Codec Type".

CSD is such an example, the encoding is defined as follows:

     8    7    6    5    4    3    2    1
  +----+----+----+----+-------------------+
  | -- | PI | PT | -- |        0xFh       |
  +----+----+----+----+-------------------+
  |      Extended Codec Type (CSData)     |
  +----+----+-----------------------------+
  | R2 | R3 |                             |
  +----+----+-----------------------------+

  CSData is coded with 0xFDh or '1111 1101' (0xfd).

Let's have the "Codec Extension" value clearly defined in the header
file, but intentionally separate from the other GSM0808_SCT_* values.

Change-Id: Iafaa25070684d2ba400c75fa33e803651a5ce857
Related: OS#6110, OS#4393, OS#4394
diff --git a/src/gsm/gsm0808.c b/src/gsm/gsm0808.c
index d115888..6ba6ef2 100644
--- a/src/gsm/gsm0808.c
+++ b/src/gsm/gsm0808.c
@@ -2478,6 +2478,7 @@
 	{ GSM0808_SCT_HR3, "HR3" },
 	{ GSM0808_SCT_HR4, "HR4" },
 	{ GSM0808_SCT_HR6, "HR6" },
+	{ GSM0808_SCT_EXT, "Codec Extension" },
 	{ GSM0808_SCT_CSD, "CSD" },
 	{ 0, NULL }
 };