avformat/ftp: use correct enum type

Fixes -Wenum-conversion from
http://fate.ffmpeg.org/report.cgi?time=20150820031140&slot=arm64-darwin-clang-apple-5.1

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/ftp.c

@@ -651,7 +651,7 @@ static int ftp_connect(URLContext *h, const char *url)
     FTPContext *s = h->priv_data;
 
     s->state = DISCONNECTED;
-    s->listing_method = UNKNOWN;
+    s->listing_method = UNKNOWN_METHOD;
     s->filesize = -1;
     s->position = 0;
     s->features = NULL;