Fix dependencies for the atempo filter.

Found, analysed and tested by trac user Jamal.

Fixes ticket #1465.

configure

@@ -1706,7 +1706,7 @@ amovie_filter_deps="avcodec avformat"
 aresample_filter_deps="swresample"
 ass_filter_deps="libass"
 asyncts_filter_deps="avresample"
-atempo_filter_deps="avcodec"
+atempo_filter_deps="avcodec rdft"
 blackframe_filter_deps="gpl"
 boxblur_filter_deps="gpl"
 colormatrix_filter_deps="gpl"