x86/EFI: avoid overrunning mb_modules[]

Commit 436fb462ab ("x86/microcode: enable boot time (pre-Dom0)
loading") added a 4th module without providing an array slot for it.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -13,7 +13,7 @@ static struct file __initdata ucode;
 static multiboot_info_t __initdata mbi = {
     .flags = MBI_MODULES | MBI_LOADERNAME
 };
-static module_t __initdata mb_modules[3];
+static module_t __initdata mb_modules[4];
 
 static void __init edd_put_string(u8 *dst, size_t n, const char *src)
 {