move gsm_04_08_gprs.h to libosmocore

This requres the corresponding commit in libosmocore.
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index 6e6b03b..111f052 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -48,7 +48,7 @@
 
 #include <openbsc/gprs_llc.h>
 #include <openbsc/gsm_04_08.h>
-#include <openbsc/gsm_04_08_gprs.h>
+#include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
 #include <openbsc/gprs_utils.h>
 
 #include <openssl/rand.h>