[jdk21u-dev] RFR: 8336413: gtk headers : Fix typedef redeclaration of GMainContext and GdkPixbuf

[Andrew Lu]

I backport this for parity with 21.0.5-oracle.

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Commit messages:
 - Backport 69baa7d2850fafbd89978db12eec683c286eb921

Changes: https://git.openjdk.org/jdk21u-dev/pull/960/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=960&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336413
  Stats: 6 lines in 2 files changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/960.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/960/head:pull/960

PR: https://git.openjdk.org/jdk21u-dev/pull/960