platform/x86: dell-rbtn: Add missing #include

Building with W=1 complains:
  CC [M]  drivers/platform/x86/dell-rbtn.o
drivers/platform/x86/dell-rbtn.c:345:5: warning: no previous prototype for 'dell_rbtn_notifier_register' [-Wmissing-prototypes]
  345 | int dell_rbtn_notifier_register(struct notifier_block *nb)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/platform/x86/dell-rbtn.c:371:5: warning: no previous prototype for 'dell_rbtn_notifier_unregister' [-Wmissing-prototypes]
  371 | int dell_rbtn_notifier_unregister(struct notifier_block *nb)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The real problem is a missing include. Add it to keep dell-rbtn.c and .h in sync.

Fixes: b05ffc95 ("dell-rbtn: Export notifier for other kernel modules")
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
[andy: massaged commit message, added Fixes tag]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>