pixdesc: mark gray8 as pseudopal

Many functions treat it as such already.
Fixes Bug 499.

CC:libav-stable@libav.org

libavutil/pixdesc.c

@@ -231,6 +231,7 @@ const AVPixFmtDescriptor av_pix_fmt_descriptors[AV_PIX_FMT_NB] = {
         .comp = {
             { 0, 0, 1, 0, 7 },        /* Y */
         },
+        .flags = PIX_FMT_PSEUDOPAL,
     },
     [AV_PIX_FMT_MONOWHITE] = {
         .name = "monow",