Encoder and Decoder Functions for SCMG messages

In the previous commit (I176b63542cbf844a2b86e01ec2534b76cf3c2f6b) the
SCMG message definitions were enabled, but no encoder/decoder functions
added.  With this change, we get {enc,dec}_PDU_SCMG_message() defined
and generated by the RAW codec.

Change-Id: I94c3918a0b63f4922bb91d6724e89ecf42e3bad9
Signed-off-by: Harald Welte <laforge@gnumonks.org>