include evdns.h through libevent.h so that libevent2 is supported too

gevent/evdns.pxi

@@ -18,7 +18,7 @@ cdef extern from "arpa/inet.h":
     int inet_pton(int af, char *src, void *dst)
     char *inet_ntop(int af, void *src, char *dst, socklen_t size)
 
-cdef extern from "evdns.h":
+cdef extern from "libevent.h":
     ctypedef void (*evdns_handler)(int result, char t, int count, int ttl,
                                    void *addrs, void *arg)
 

gevent/libevent.h

@@ -12,6 +12,8 @@
 #include "event2/http_struct.h"
 #include "event2/buffer.h"
 #include "event2/buffer_compat.h"
+#include "event2/dns.h"
+#include "event2/dns_compat.h"
 
 #define EVBUFFER_DRAIN evbuffer_drain
 #define EVHTTP_SET_CB  evhttp_set_cb
@@ -22,6 +24,7 @@
 
 #include "event.h"
 #include "evhttp.h"
+#include "evdns.h"
 
 /* compatibility */