PM: Use proper ccflag flag in kernel/power/Makefile

Use the ccflags-$ flag instead of EXTRA_CFLAGS because EXTRA_CFLAGS is
deprecated and should now be switched.  According to
(documentation/kbuild/makefiles.txt).

Signed-off-by: Tracey Dent <tdent48227@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>