contrib/buildsystems: ignore irrelevant files in Generators/

The Generators/ directory can contain spurious files such as editors'
backup files. Even worse, there could be .swp files which are not even
valid Perl scripts.

Let's just ignore anything but .pm files in said directory.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

contrib/buildsystems/Generators.pm

@@ -17,7 +17,7 @@ BEGIN {
     $me = dirname($me);
     if (opendir(D,"$me/Generators")) {
         foreach my $gen (readdir(D)) {
-            next if ($gen  =~ /^\.\.?$/);
+            next unless ($gen  =~ /\.pm$/);
             require "${me}/Generators/$gen";
             $gen =~ s,\.pm,,;
             push(@AVAILABLE, $gen);