configure: fix compilation without libmodplug

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -1497,7 +1497,7 @@ libgsm_decoder_deps="libgsm"
 libgsm_encoder_deps="libgsm"
 libgsm_ms_decoder_deps="libgsm"
 libgsm_ms_encoder_deps="libgsm"
-libmodplug_decode_deps="libmodplug"
+libmodplug_demuxer_deps="libmodplug"
 libmp3lame_encoder_deps="libmp3lame"
 libopencore_amrnb_decoder_deps="libopencore_amrnb"
 libopencore_amrnb_encoder_deps="libopencore_amrnb"