libnetlink: make ipset.h self-contained

We need to include <netinet/in.h> to define 'in_addr_t'.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/libnetlink/ipset.h

 #ifndef __IPSET_H
 #define __IPSET_H
 
+#include <netinet/in.h>
+
 int ipset_add(const char *name, in_addr_t addr);
 int ipset_del(const char *name, in_addr_t addr);
 int ipset_flush(const char *name);