ANDROID: GKI: sched.h: add Android ABI padding to some structures

Try to mitigate potential future driver core api changes by adding a
pointer to struct signal_struct, struct sched_entity, struct
sched_rt_entity, and struct task_struct.

Based on a patch from Michal Marek <mmarek@suse.cz> from the SLES kernel

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I1449735b836399e9b356608727092334daf5c36b