platform/x86: fix dell-laptop function prototypes

Fix build warnings:
  drivers/platform/x86/dell-laptop.c:592:13: warning: function declaration isn't a prototype
  drivers/platform/x86/dell-laptop.c:599:13: warning: function declaration isn't a prototype

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>