configure, libavformat/Makefile: Fix webm_chunk dependencies

The webm_chunk muxer requires the WebM muxer, yet it does not directly
require anything from libavformat/matroska.c (it does not even include
the corresponding header). So remove the dependency from the Makefile
and add a _select to configure.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

configure

@@ -3336,6 +3336,7 @@ w64_demuxer_select="wav_demuxer"
 w64_muxer_select="wav_muxer"
 wav_demuxer_select="riffdec"
 wav_muxer_select="riffenc"
+webm_chunk_muxer_select="webm_muxer"
 webm_muxer_select="iso_media riffenc"
 webm_dash_manifest_demuxer_select="matroska_demuxer"
 wtv_demuxer_select="mpegts_demuxer riffdec"

libavformat/Makefile

@@ -555,7 +555,7 @@ OBJS-$(CONFIG_WEBM_MUXER)                += matroskaenc.o matroska.o \
                                             wv.o vorbiscomment.o \
                                             webmdashenc.o webm_chunk.o
 OBJS-$(CONFIG_WEBM_DASH_MANIFEST_MUXER)  += webmdashenc.o matroska.o
-OBJS-$(CONFIG_WEBM_CHUNK_MUXER)          += webm_chunk.o matroska.o
+OBJS-$(CONFIG_WEBM_CHUNK_MUXER)          += webm_chunk.o
 OBJS-$(CONFIG_WEBP_MUXER)                += webpenc.o
 OBJS-$(CONFIG_WEBVTT_DEMUXER)            += webvttdec.o subtitles.o
 OBJS-$(CONFIG_WEBVTT_MUXER)              += webvttenc.o