Add *.j2k extension to JPEG2000 decoder

Some files with *.j2k extension can be found for example here:

http://www.openjpeg.org/index.php?menu=samplesSigned-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/img2.c

@@ -75,6 +75,7 @@ static const IdStrMap img_tags[] = {
     { CODEC_ID_SUNRAST   , "im8"},
     { CODEC_ID_SUNRAST   , "im24"},
     { CODEC_ID_SUNRAST   , "sunras"},
+    { CODEC_ID_JPEG2000  , "j2k"},
     { CODEC_ID_JPEG2000  , "jp2"},
     { CODEC_ID_DPX       , "dpx"},
     { CODEC_ID_PICTOR    , "pic"},