fvp: fix prerequisites of DTB target

The output directory shall be created before the DTB target runs.

Change-Id: Iaf6b6d5d484df2b4784c0ef115764d36d5a626c8
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
diff --git a/fvp.mk b/fvp.mk
index 15b2643..bf477f5 100644
--- a/fvp.mk
+++ b/fvp.mk
@@ -220,7 +220,7 @@
 DTS_PRE		?= $(OUT_PATH)/$(DTS).pre.dts
 DTB		?= $(OUT_PATH)/$(DTS).dtb
 
-$(DTB): $(DTS_PATH)/$(DTS).dts
+$(DTB): $(DTS_PATH)/$(DTS).dts $(OUT_PATH)
 	gcc -E -P -nostdinc -undef -x assembler-with-cpp -I$(DTSI_PATH) \
 		-I$(TF_A_PATH)/include -o $(DTS_PRE) $(DTS_PATH)/$(DTS).dts
 	dtc -i$(DTSI_PATH) -I dts -O dtb -o $(DTB) $(DTS_PRE)