RFR: JDK-8336413 : gtk headers : Fix typedef redeclaration of GMainContext and GdkPixbuf

[Harshitha Onkar]

Typedef re-declaration warnings were seen on Solaris in 11u but this seems to be an issue in all releases including mainline, but only the Solaris compiler warns. 
 
Two typedef declarations are causing this issue - `GMainContext, GdkPixbuf`

The problem is that they are defined in gtk_interface.h but then redefined in gtk2_interface.h and gtk3_interface.h
The fix is to remove the redeclarations in gtk2_interface.h and gtk3_interface.h

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Commit messages:
 - typedef redeclaration removed for GdkPixbuf, GMainContext

Changes: https://git.openjdk.org/jdk/pull/20186/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20186&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/jdk/pull/20186.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20186/head:pull/20186

PR: https://git.openjdk.org/jdk/pull/20186