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

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

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

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I5432e05386265281d993199599c6f9dcd17a9daf