x86: dell-laptop: depends on POWER_SUPPLY

Build breaks when DELL_LAPTOP=y and POWER_SUPPLY=m.  DELL_LAPTOP needs to
depend on POWER_SUPPLY.

dell-laptop.c:(.text+0x1ef3c4): undefined reference to `power_supply_is_system_supplied'
dell-laptop.c:(.text+0x1ef45e): undefined reference to `power_supply_is_system_supplied'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/platform/x86/Kconfig

@@ -62,6 +62,7 @@ config DELL_LAPTOP
 	depends on EXPERIMENTAL
 	depends on BACKLIGHT_CLASS_DEVICE
 	depends on RFKILL
+	depends on POWER_SUPPLY
 	default n
 	---help---
 	This driver adds support for rfkill and backlight control to Dell