Add a lavc Makefile dependency for the elbg filter.

Fixes compilation with --disable-encoders

Reported by trac user Julian in ticket #3140.

libavcodec/Makefile

@@ -687,6 +687,9 @@ OBJS-$(CONFIG_WEBM_MUXER)              += mpeg4audio.o mpegaudiodata.o  \
                                           vorbis_data.o
 OBJS-$(CONFIG_WTV_DEMUXER)             += mpeg4audio.o mpegaudiodata.o
 
+# libavfilter dependencies
+OBJS-$(CONFIG_ELBG_FILTER)             += elbg.o
+
 # external codec libraries
 OBJS-$(CONFIG_LIBAACPLUS_ENCODER)         += libaacplus.o
 OBJS-$(CONFIG_LIBCELT_DECODER)            += libcelt_dec.o