build: Add missing object file for Matroska demuxer

Fixes compilation with --disable-everything --disable-network --enable-demuxer=matroska
Signed-off-by: Carl Eugen Hoyos <cehoyos@ag.or.at>

libavformat/Makefile

@@ -208,7 +208,8 @@ OBJS-$(CONFIG_M4V_DEMUXER)               += m4vdec.o rawdec.o
 OBJS-$(CONFIG_M4V_MUXER)                 += rawenc.o
 OBJS-$(CONFIG_MATROSKA_DEMUXER)          += matroskadec.o matroska.o  \
                                             isom.o rmsipr.o flac_picture.o \
-                                            oggparsevorbis.o vorbiscomment.o
+                                            oggparsevorbis.o vorbiscomment.o \
+                                            replaygain.o
 OBJS-$(CONFIG_MATROSKA_MUXER)            += matroskaenc.o matroska.o \
                                             isom.o avc.o hevc.o \
                                             flacenc_header.o avlanguage.o vorbiscomment.o wv.o