Fix missing preview images for actions when browsing in external files

When reading the preview images of external .blend files, action data-blocks
were not handled.
Preview support for actions was added in 2397ccc583.

source/blender/blenloader/intern/readblenentry.c

@@ -243,6 +243,7 @@ LinkNode *BLO_blendhandle_get_previews(BlendHandle *bh, int ofblocktype, int *to
         case ID_OB:  /* fall through */
         case ID_GR:  /* fall through */
         case ID_SCE: /* fall through */
+        case ID_AC:  /* fall through */
           new_prv = MEM_callocN(sizeof(PreviewImage), "newpreview");
           BLI_linklist_prepend(&previews, new_prv);
           tot++;