dix: FindBestPixel: fix implicit fallthrough warning

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1673>
(cherry picked from commit 9c9e1afeb2)

dix/colormap.c

@@ -1257,6 +1257,7 @@ FindBestPixel(EntryPtr pentFirst, int size, xrgb * prgb, int channel)
         case PSEUDOMAP:
             dg = (long) pent->co.local.green - prgb->green;
             db = (long) pent->co.local.blue - prgb->blue;
+            /* fallthrough */
         case REDMAP:
             dr = (long) pent->co.local.red - prgb->red;
             break;