PM / sleep: Move prototype declaration to header file kernel/power/power.h

Move prototype declaration of function to header file
kernel/power/power.h because it is used by more than one file.

This eliminates the following warning in kernel/power/snapshot.c:

kernel/power/snapshot.c:1588:16: warning: no previous prototype for ‘swsusp_save’ [-Wmissing-prototypes]
Signed-off-by: NRashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/power.h

@@ -49,6 +49,8 @@ static inline char *check_image_kernel(struct swsusp_info *info)
  */
 #define SPARE_PAGES	((1024 * 1024) >> PAGE_SHIFT)
 
+asmlinkage int swsusp_save(void);
+
 /* kernel/power/hibernate.c */
 extern bool freezer_test_done;