avserver: move avserver-specific code from ffmdec.c to avserver.c

This way avserver only depends on the data structures of the ffm
demuxer, which it already does, and not also on private functions
being exported by the library.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/ffm.h

@@ -56,8 +56,4 @@ typedef struct FFMContext {
     uint8_t packet[FFM_PACKET_SIZE];
 } FFMContext;
 
-int64_t ffm_read_write_index(int fd);
-int ffm_write_write_index(int fd, int64_t pos);
-void ffm_set_write_index(AVFormatContext *s, int64_t pos, int64_t file_size);
-
 #endif /* AVFORMAT_FFM_H */