tools/copyright.pl: skip copyright changes for *.key files

Reported-by: Alvaro Herrera

Discussion: https://postgr.es/m/20200102184059.GA25435@alvherre.pgsql

Backpatch-through: master

src/tools/copyright.pl

@@ -41,7 +41,7 @@ sub wanted
 
 	# skip file names with binary extensions
 	# How are these updated?  bjm 2012-01-02
-	return if ($_ =~ m/\.(ico|bin|po)$/);
+	return if ($_ =~ m/\.(ico|bin|po|key)$/);
 
 	my @lines;
 	tie @lines, "Tie::File", $File::Find::name;