RFR: JDK-8261654: Missing license header in Signatures.java

[Hannes Wallnöfer]

License header was missing in src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/Signatures.java.

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Commit messages:
 - JDK-8261654: Missing license header in Signatures.java

Changes: https://git.openjdk.java.net/jdk/pull/2549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261654
  Stats: 25 lines in 1 file changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2549.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2549/head:pull/2549

PR: https://git.openjdk.java.net/jdk/pull/2549