Use gsm48_encode_ra() for RAI encoding

It has stricter type signature which increase the chance of spotting
misuse either via compiler warning or with automated scan. This also
paves the way for gsm48_construct_ra() deprecation in libosmocore.

Change-Id: I2c0f082dc7214ed57a40dad0788e34b838dfac97
Related: OS#1640
diff --git a/src/gprs/sgsn_libgtp.c b/src/gprs/sgsn_libgtp.c
index 1032230..ae293f1 100644
--- a/src/gprs/sgsn_libgtp.c
+++ b/src/gprs/sgsn_libgtp.c
@@ -268,7 +268,7 @@
 	raid = mmctx->ra;
 	raid.lac = 0xFFFE;
 	raid.rac = 0xFF;
-	gsm48_construct_ra(pdp->rai.v, &raid);
+	gsm48_encode_ra((struct gsm48_ra_id *)pdp->rai.v, &raid);
 
 	/* Encode User Location Information accordint to TS 29.060 7.7.51 */
 	pdp->userloc_given = 1;