RFR: 8283457: [macos] libpng build failures with Xcode13.3

[Phil Race]

Disable a warning from the Xcode 13.3 clang compiler when compiling libpng, which is used by libsplashscreen.

Policy is not to change the upstream code locally unless it is also changed in the same way upstream.
We could consider reporting it upstream but libpng releases are very infrequent.

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Commit messages:
 - 8283457: [macos] libpng build failures with Xcode13.3

Changes: https://git.openjdk.java.net/jdk/pull/7911/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7911&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283457
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7911.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7911/head:pull/7911

PR: https://git.openjdk.java.net/jdk/pull/7911