libavformat: Build hevc.o when building the RTP muxer

The RTP muxer enables the actual codepaths within sdp.c,
which depend on hevc.o since e5cfc8fd.

This fixes builds with --disable-everything --enable-muxer=rtp.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/Makefile

@@ -299,7 +299,7 @@ OBJS-$(CONFIG_RTP_MUXER)                 += rtp.o         \
                                             rtpenc_h264.o \
                                             rtpenc_vp8.o  \
                                             rtpenc_xiph.o \
-                                            avc.o
+                                            avc.o hevc.o
 OBJS-$(CONFIG_RTSP_DEMUXER)              += rtsp.o rtspdec.o httpauth.o \
                                             urldecode.o
 OBJS-$(CONFIG_RTSP_MUXER)                += rtsp.o rtspenc.o httpauth.o \