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/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index 062de44..dde1e5e 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -48,7 +48,7 @@
 #include <openbsc/gprs_llc.h>
 #include <openbsc/gprs_sgsn.h>
 #include <openbsc/gprs_gmm.h>
-#include <openbsc/gsm_subscriber.h>
+#include <openbsc/gprs_subscriber.h>
 #include <openbsc/gprs_sndcp.h>
 
 #ifdef BUILD_IU