efi: Make EFI_SECURE_BOOT_SIG_ENFORCE depend on EFI

BugLink: http://bugs.launchpad.net/bugs/1566221

The functionality of the config option is dependent upon the platform being
UEFI based.  Reflect this in the config deps.
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

arch/x86/Kconfig

@@ -1722,6 +1722,7 @@ config EFI_MIXED
 
 config EFI_SECURE_BOOT_SIG_ENFORCE
 	def_bool n
+	depends on EFI
 	prompt "Force module signing when UEFI Secure Boot is enabled"
 	---help---
 	  UEFI Secure Boot provides a mechanism for ensuring that the