Ignore errors from the File::LibMagic describe_filename function.

Newer versions of amavisd-new can make use of the File::LibMagic
library (as opposed to the "file" executable) to get information about
attachments. Using the library should be faster, but when it fails, it
does so in a novel way:

  (02859-21) File::LibMagic::describe_filename failed on p003: libmagic
  JPEG image data, Exif standard: [TIFF image data, big-endian,...

This commit adds such lines to the global ignore list, since they
don't provide us with any useful information.