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>