gb: suspend_ref was changed to a plain uint8_t, update the code

Require a 0.5.2+ libosmogb due the change of the suspend_ref type.
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index d1df094..7d80dc8 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -119,7 +119,7 @@
 			return gprs_gmm_rx_suspend(bp->ra_id, bp->tlli);
 		case PRIM_BSSGP_GMM_RESUME:
 			return gprs_gmm_rx_resume(bp->ra_id, bp->tlli,
-						  *bp->u.resume.suspend_ref);
+						  bp->u.resume.suspend_ref);
 		}
 		break;
 	case SAP_BSSGP_NM: