RFR: JDK-8239378 Add Classpath Exception to license in source file.

[Jonathan Gibbons]

Please review a small change to add "Classpath Exception" to a file.

The file itself is not actually used in the JDK build; it is just thge 
reference for a minified copy in another file.

JBS: https://bugs.openjdk.java.net/browse/JDK-8239378
Patch: inline below

-- Jon

$ hg diff -R open open/src/jdk.javadoc/share/classes/
diff -r a614219d7388 
src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/resources/external-link.svg
--- 
a/src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/resources/external-link.svg 
Tue Feb 18 22:25:08 2020 +0100
+++ 
b/src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/resources/external-link.svg 
Tue Feb 18 13:47:02 2020 -0800
@@ -6,7 +6,9 @@

   This code is free software; you can redistribute it and/or modify it
   under the terms of the GNU General Public License version 2 only, as
- published by the Free Software Foundation.
+ published by the Free Software Foundation.  Oracle designates this
+ particular file as subject to the "Classpath" exception as provided
+ by Oracle in the LICENSE file that accompanied this code.

   This code is distributed in the hope that it will be useful, but WITHOUT
   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or