Soften the desktop-file-utils message.

Some .desktop files don't have a MimeType= property and thus do not
require desktop-file-utils.  Since we're just looking at the port bits,
we can't know for certain if this error is valid.

PR:	271623

portlint.pl

@@ -810,8 +810,9 @@ sub checkplist {
 		}
 
 		if ($_ =~ m|\.desktop$| && $makevar{USES} !~ /\bdesktop-file-utils\b/) {
-			&perror("FATAL", $file, $., "this port installs .desktop files. ".
-				"Please add `desktop-file-utils` to USES.");
+			&perror("WARN", $file, $., "this port installs .desktop files. ".
+				"If the .desktop file(s) installed contain ``MimeType='', ".
+				"you must add `desktop-file-utils` to USES.");
 		}
 
 		if ($_ =~ m|^(%%([^%]+)%%)?.*\.mo$| && $makevar{USES} !~ /\bgettext\b/) {