Fix T39186: Matroska audio support (mka, mkv without video stream)

.mka files were considered image files when adding to sequencer
using drag-n-drop.

source/blender/imbuf/intern/util.c

@@ -179,6 +179,7 @@ const char *imb_ext_audio[] = {
 	".aif",
 	".aiff",
 	".m4a",
+	".mka",
 	NULL
 };