move gsm_04_08_gprs.h to libosmocore

This requres the corresponding commit in libosmocore.
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 678c1de..5b0bf39 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -20,6 +20,7 @@
  *
  */
 
+#include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
 #include <openbsc/gsm_subscriber.h>
 #include <openbsc/gprs_gsup_client.h>