rename gsm0408_gprs_rcvmsg() to gsm0408_gprs_rcvmsg_gb()

This is the entry point for GMM from Gb.  We will create a new one
for Iu, so let's be explicit rather than implicit.

Change-Id: I93c074bf99db041117c0dc03dc8255879845a875
diff --git a/openbsc/src/gprs/gprs_llc.c b/openbsc/src/gprs/gprs_llc.c
index 4d21963..71178c9 100644
--- a/openbsc/src/gprs/gprs_llc.c
+++ b/openbsc/src/gprs/gprs_llc.c
@@ -663,7 +663,7 @@
 		switch (llhp.sapi) {
 		case GPRS_SAPI_GMM:
 			/* send LL_UNITDATA_IND to GMM */
-			rc = gsm0408_gprs_rcvmsg(msg, lle->llme);
+			rc = gsm0408_gprs_rcvmsg_gb(msg, lle->llme);
 			break;
 		case GPRS_SAPI_SNDCP3:
 		case GPRS_SAPI_SNDCP5: