GPRS: Increase the NS msg size to 2048

According to the GPRS NS spec the maximum framesize
is 1600 octets for FrameRelay, it can be bigger if
configured to be so. Make it 2048 octets to have some
space available...
diff --git a/openbsc/include/openbsc/gprs_ns.h b/openbsc/include/openbsc/gprs_ns.h
index 8b5e7dc..953c364 100644
--- a/openbsc/include/openbsc/gprs_ns.h
+++ b/openbsc/include/openbsc/gprs_ns.h
@@ -222,7 +222,7 @@
 /* Add NS-specific VTY stuff */
 int gprs_ns_vty_init(struct gprs_ns_inst *nsi);
 
-#define NS_ALLOC_SIZE	1024
+#define NS_ALLOC_SIZE	2048
 #define NS_ALLOC_HEADROOM 20
 static inline struct msgb *gprs_ns_msgb_alloc(void)
 {