<AWT Dev> [9] Review Request: 8030987 sun_awt_X11_GtkFileDialogPeer.h can be removed

[Sergey Bylokhov]

Hello.
Please review the fix for jdk 9.
  - sun_awt_X11_GtkFileDialogPeer.h was removed, because it is created 
during the build of jdk.
  - sun_awt_X11_GtkFileDialogPeer.c added copyright header
  - small cleanup in java file.

Bug: https://bugs.openjdk.java.net/browse/JDK-8030987
Webrev can be found at: http://cr.openjdk.java.net/~serb/8030987/webrev.00

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Best regards, Sergey.