RFR: 8344155: Add missing @Override annotation to GIFImageLoader2

[Michael Strauß]

The method `load(...)` is missing an `@Override` annotation.

A single reviewer is sufficient.

-------------

Commit messages:
 - Add missing Override

Changes: https://git.openjdk.org/jfx/pull/1636/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1636&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344155
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1636.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1636/head:pull/1636

PR: https://git.openjdk.org/jfx/pull/1636