move gsm_04_08_gprs.h to libosmocore

This requres the corresponding commit in libosmocore.
diff --git a/openbsc/src/gprs/sgsn_auth.c b/openbsc/src/gprs/sgsn_auth.c
index b83294d..f0c4316 100644
--- a/openbsc/src/gprs/sgsn_auth.c
+++ b/openbsc/src/gprs/sgsn_auth.c
@@ -19,11 +19,11 @@
  *
  */
 
+#include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
 #include <openbsc/sgsn.h>
 #include <openbsc/gprs_sgsn.h>
 #include <openbsc/gprs_gmm.h>
 #include <openbsc/gsm_subscriber.h>
-#include <openbsc/gsm_04_08_gprs.h>
 #include <openbsc/debug.h>
 
 const struct value_string auth_state_names[] = {