PM / Domains: fix compilation failure for CONFIG_PM_GENERIC_DOMAINS unset

Fix the following compalitaion breakage:

In file included from linux/drivers/sh/pm_runtime.c:15:
linux/include/linux/pm_domain.h: In function 'dev_to_genpd':
linux/include/linux/pm_domain.h:142: error: implicit declaration of function 'ERR_PTR'
linux/include/linux/pm_domain.h:142: warning: return makes pointer from integer without a cast
In file included from linux/include/linux/sh_clk.h:10,
                 from linux/drivers/sh/pm_runtime.c:19:
linux/include/linux/err.h: At top level:
linux/include/linux/err.h:22: error: conflicting types for 'ERR_PTR'
linux/include/linux/pm_domain.h:142: note: previous implicit declaration of 'ERR_PTR' was here
make[3]: *** [drivers/sh/pm_runtime.o] Error 1

Reported-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>