add struct gprs_subscr, separating gprs from gsm_subscriber

Prepare for replacing gsm_subscriber with vlr_subscriber. vlr_subscriber will
not make sense to be used in gprs, so have a dedicated GPRS subscriber struct.
(Could change if the gprs code were to use libvlr; is currently independent).

Related: OS#1592
Change-Id: Ia8b391ee009c8545763cba04505be3947835120e
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 260e032..727524e 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -31,7 +31,7 @@
 #include <osmocom/gprs/gprs_bssgp.h>
 #include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
 
-#include <openbsc/gsm_subscriber.h>
+#include <openbsc/gprs_subscriber.h>
 #include <openbsc/debug.h>
 #include <openbsc/gprs_sgsn.h>
 #include <openbsc/sgsn.h>
@@ -321,9 +321,9 @@
 
 	/* Detach from subscriber which is possibly freed then */
 	if (mm->subscr) {
-		struct gsm_subscriber *subscr = subscr_get(mm->subscr);
+		struct gprs_subscr *subscr = gprs_subscr_get(mm->subscr);
 		gprs_subscr_cleanup(subscr);
-		subscr_put(subscr);
+		gprs_subscr_put(subscr);
 	}
 
 	sgsn_mm_ctx_free(mm);