Add '.so' to the list of file extensions cleared up by 'make clean' in

Makefile.cyg, since if you're building against Winelib it will
generate one of those alongside each .exe file.

[originally from svn r9952]

mkfiles.pl

@@ -492,7 +492,7 @@ if (defined $makefiles{'cygwin'}) {
     print "\n";
     print $makefile_extra{'cygwin'}->{'end'};
     print "\nclean:\n".
-    "\trm -f *.o *.exe *.res.o *.map\n".
+    "\trm -f *.o *.exe *.res.o *.so *.map\n".
     "\n".
     "FORCE:\n";
     select STDOUT; close OUT;