ggsn: Send proper errors in create_context_ind()

When we receive PDP context requests for unknown PDP types or if
we run out of dynamic addresses, we need to inform the SGSN that
PDP context creation failed.

Change-Id: Ibf199c1726130d27c8f80230b30ee51101c93b06
diff --git a/lib/ippool.h b/lib/ippool.h
index e3c1c92..ba92a56 100644
--- a/lib/ippool.h
+++ b/lib/ippool.h
@@ -13,6 +13,7 @@
 #define _IPPOOL_H
 
 #include "../lib/in46_addr.h"
+#include "../gtp/gtp.h"
 
 /* Assuming that the address space is fragmented we need a hash table
    in order to return the addresses.