Fix obviously missing dependency of float DCT.

Fixes issue 2095.

Originally committed as revision 24245 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1160,6 +1160,7 @@ symver_if_any="symver_asm_label symver_gnu_asm"
 # subsystems
 mdct_select="fft"
 rdft_select="fft"
+dct_select="rdft"
 
 # decoders / encoders / hardware accelerators
 aac_decoder_select="mdct rdft"