crypto/ec/asm/x25519-x86_64.pl: close STDOUT, not $STDOUT

Fixes #5471
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5472)

crypto/ec/asm/x25519-x86_64.pl

@@ -815,4 +815,4 @@ ___
 
 $code =~ s/\`([^\`]*)\`/eval $1/gem;
 print $code;
-close $STDOUT;
+close STDOUT;