Fix the dependencies of the RTP muxer

This fixes building with --disable-everything --enable-muxer=rtp, closing
issue 2159.

Originally committed as revision 25036 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -522,7 +522,7 @@ OBJS-$(CONFIG_NUT_MUXER)               += mpegaudiodata.o
 OBJS-$(CONFIG_OGG_DEMUXER)             += flacdec.o flacdata.o flac.o \
                                           dirac.o mpeg12data.o
 OBJS-$(CONFIG_OGG_MUXER)               += xiph.o flacdec.o flacdata.o flac.o
-OBJS-$(CONFIG_RTP_MUXER)               += mpegvideo.o
+OBJS-$(CONFIG_RTP_MUXER)               += mpegvideo.o xiph.o
 OBJS-$(CONFIG_WEBM_MUXER)              += xiph.o mpeg4audio.o \
                                           flacdec.o flacdata.o flac.o