ANDROID: GKI: timer.h: add Android ABI padding to a structure

Try to mitigate potential future driver core api changes by adding a
padding to struct timer_list.

Based on a change made to the RHEL/CENTOS 8 kernel.

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I74673e2271cd757c4871c9bcb69fd73bb22a722b