check_installed: Don't show error for mxtoools

mxtools-0.0-0 is registered with the bee index but there is no bee file
for it, neither in this repository not in the system.

So scripts/ckeck_installed throws a warning

    buczek@deinemuddah:~/git/bee-files (master)$ scripts/check-installed
    # no such file /usr/share/bee/mxtools-0.0-0/mxtools-0.0-0

Avoid the warning for the /mxtools-0.0-0 virtual package.

scripts/check-installed

@@ -57,6 +57,7 @@ for my $beefile (scandir('.')) {
 
 for my $dir (scandir('/usr/share/bee')) {
 	my $pkg=$dir;
+	$pkg eq 'mxtools-0.0-0' and next;
 	my $beefile=save_qx('beeversion','--format','%F',"$pkg.bee");
 	unless (-e "/usr/share/bee/$pkg/$beefile.bee") {
 		print "# no such file /usr/share/bee/$pkg/$beefile\n";