lavf: hide private symbols.

Overhead as reported by rbelf-size goes from 40147 to 20877.

libavformat/libavformat.v

 LIBAVFORMAT_$MAJOR {
-        global: *;
-        local:
-                ff_*_demuxer;
-                ff_*_muxer;
-                ff_*_protocol;
+        global: av*;
+                #FIXME those are for avserver
+                ff_inet_aton;
+                ff_socket_nonblock;
+                ffm_set_write_index;
+                ffm_read_write_index;
+                ffm_write_write_index;
+                ff_rtsp_parse_line;
+                ff_rtp_get_local_rtp_port;
+                ff_rtp_get_local_rtcp_port;
+                ffio_open_dyn_packet_buf;
+                url_open;
+                url_close;
+                url_write;
+                url_get_max_packet_size;
+                #those are deprecated, remove on next bump
+                find_info_tag;
+                parse_date;
+                dump_format;
+                url_*;
+        local: *;
 };