x86/Kconfig: Make EFI select UCS2_STRING

The commit "efi: Distinguish between "remaining space" and actually used
space" added usage of ucs2_*() functions to arch/x86/platform/efi/efi.c,
but the only thing which selected UCS2_STRING was EFI_VARS, which is
technically optional and can be built as a module.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/x86/Kconfig

@@ -1549,6 +1549,7 @@ config X86_SMAP
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
+	select UCS2_STRING
 	---help---
 	  This enables the kernel to use EFI runtime services that are
 	  available (such as the EFI variable services).