The dxa demuxer depends on riff.o.

This fixes the following option set:
  --disable-encoders --disable-decoders --disable-parsers --disable-bsfs
  --disable-muxers --disable-demuxers --disable-protocols --enable-demuxer=dxa

Originally committed as revision 9647 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/Makefile

@@ -41,7 +41,7 @@ OBJS-$(CONFIG_DTS_DEMUXER)               += raw.o
 OBJS-$(CONFIG_DV_DEMUXER)                += dv.o
 OBJS-$(CONFIG_DV_MUXER)                  += dvenc.o
 OBJS-$(CONFIG_DV1394_DEMUXER)            += dv1394.o dv.o
-OBJS-$(CONFIG_DXA_DEMUXER)               += dxa.o
+OBJS-$(CONFIG_DXA_DEMUXER)               += dxa.o riff.o
 OBJS-$(CONFIG_EA_DEMUXER)                += electronicarts.o
 OBJS-$(CONFIG_FFM_DEMUXER)               += ffm.o
 OBJS-$(CONFIG_FFM_MUXER)                 += ffm.o