PM / domains: Fix 'may be used uninitialized' build warning

This patch fixes the following gcc warning:

drivers/base/power/domain.c: In function ‘genpd_runtime_resume’:
drivers/base/power/domain.c:642:14: warning: ‘time_start’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
   elapsed_ns = ktime_to_ns(ktime_sub(ktime_get(), time_start)

The same problem (in another function in this same file) was fixed in
commit d33d5a6c (avoid spurious "may be used uninitialized" warning)

Signed-off-by: Augusto Mecking Caringi <augustocaringi@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>