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/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index 4425a10..a14e59f 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -26,6 +26,8 @@
 
 #define OBSC_LINKID_CB(__msgb)	(__msgb)->cb[3]
 
+#define tmsi_from_string(str) strtoul(str, NULL, 10)
+
 enum gsm_security_event {
 	GSM_SECURITY_NOAVAIL,
 	GSM_SECURITY_AUTH_FAILED,