arch: arm64: Include the device tree library when X0_DTB is set

Bug: 297275002
Test: Build + run Trusty

Change-Id: I9d020aed1a08286933b698cf6c642c76b1183822
diff --git a/arch/arm64/rules.mk b/arch/arm64/rules.mk
index f994e8f..d20f3c5 100644
--- a/arch/arm64/rules.mk
+++ b/arch/arm64/rules.mk
@@ -82,7 +82,7 @@
 $(error unrecognized ARM64_BOOT_PROTOCOL, $(ARM64_BOOT_PROTOCOL), not in [$(ARM64_BOOT_PROTOCOLS)])
 endif
 
-ifeq (X0_MEMSIZE,$(ARM64_BOOT_PROTOCOL))
+ifeq (X0_DTB,$(ARM64_BOOT_PROTOCOL))
 MODULE_DEPS += \
 	$(LKROOT)/lib/device_tree \