Don't try to import a non-exported object in vcregress.pl

Commit ca9e9b08e4 wrongly tried to import devnull from File::Spec, but
it's not exported, you just call the method via the class. This was
harmless until modern perls complained, so stop doing that.

Per buildfarm failures.

Backpatch 14 thru 16

src/tools/msvc/vcregress.pl

@@ -14,7 +14,7 @@ use File::Basename;
 use File::Copy;
 use File::Find ();
 use File::Path qw(rmtree);
-use File::Spec qw(devnull);
+use File::Spec;
 
 use FindBin;
 use lib $FindBin::RealBin;