x86_64-gf2m.pl: add missing Windows build fix for #2963.

PR: 3004

crypto/bn/asm/x86_64-gf2m.pl

@@ -31,7 +31,8 @@ $0 =~ m/(.*[\/\\])[^\/\\]+$/; $dir=$1;
 ( $xlate="${dir}../../perlasm/x86_64-xlate.pl" and -f $xlate) or
 die "can't locate x86_64-xlate.pl";
 
-open STDOUT,"| \"$^X\" $xlate $flavour $output";
+open OUT,"| \"$^X\" $xlate $flavour $output";
+STDOUT=*OUT;
 
 ($lo,$hi)=("%rax","%rdx");	$a=$lo;
 ($i0,$i1)=("%rsi","%rdi");