configure: Add fork as a dependency to ffserver

configure

@@ -1471,7 +1471,7 @@ ffmpeg_select="buffer_filter"
 ffplay_deps="avcodec avformat swscale sdl"
 ffplay_select="rdft"
 ffprobe_deps="avcodec avformat"
-ffserver_deps="avformat ffm_muxer rtp_protocol rtsp_demuxer"
+ffserver_deps="avformat ffm_muxer fork rtp_protocol rtsp_demuxer"
 ffserver_extralibs='$ldl'
 
 doc_deps="texi2html"