package/ffmpeg: add optional support for libxml2

Support for libxml2 was added 2017 in ffmpeg version 3.4:
http://git.videolan.org/?p=ffmpeg.git;a=commit;h=96d70694aea64616c68db8be306c159c73fb3980

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/ffmpeg/ffmpeg.mk

@@ -383,6 +383,13 @@ else
 FFMPEG_CONF_OPTS += --disable-iconv
 endif
 
+ifeq ($(BR2_PACKAGE_LIBXML2),y)
+FFMPEG_CONF_OPTS += --enable-libxml2
+FFMPEG_DEPENDENCIES += libxml2
+else
+FFMPEG_CONF_OPTS += --disable-libxml2
+endif
+
 # ffmpeg freetype support require fenv.h which is only
 # available/working on glibc.
 # The microblaze variant doesn't provide the needed exceptions