PM: Fix freezer compilation if PM_SLEEP is unset

Freezer fails to compile if with the following configuration
settings:

CONFIG_CGROUPS=y
CONFIG_CGROUP_FREEZER=y
CONFIG_MODULES=y
CONFIG_FREEZER=y
CONFIG_PM=y
CONFIG_PM_SLEEP=n

Fix this by making process.o compilation depend on CONFIG_FREEZER.

Reported-by: Cheng Renquan <crquan@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Len Brown <len.brown@intel.com>