add proper matroskaenc dependency over xiph.o

Originally committed as revision 11580 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -299,6 +299,7 @@ OBJS-$(CONFIG_ADPCM_YAMAHA_DECODER)    += adpcm.o
 OBJS-$(CONFIG_ADPCM_YAMAHA_ENCODER)    += adpcm.o
 
 # libavformat dependencies
+OBJS-$(CONFIG_MATROSKA_MUXER)          += xiph.o
 OBJS-$(CONFIG_OGG_MUXER)               += xiph.o
 
 # external codec libraries