ANDROID: GKI: genirq: export `kstat_irqs_usr` for watchdog

Export `kstat_irqs_usr` for vendor irq tracking statistics.

Bug: 172988823
Bug: 176525217
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: Ib3bddbbe9d252be79f3222e941e1cde1625b793a
Signed-off-by: Will McVicker <willmcvicker@google.com>