Add a build dependency for ramdisk.img on system/core/rootdir/init.rc

Change-Id: Id9698a88720cc0c70e3d9c43c9bac9b74f4798b6
diff --git a/rootdir/Android.mk b/rootdir/Android.mk
index 7ac991c..ed13060 100644
--- a/rootdir/Android.mk
+++ b/rootdir/Android.mk
@@ -37,6 +37,7 @@
 $(file) : $(LOCAL_PATH)/init.rc | $(ACP)
 	$(transform-prebuilt-to-target)
 ALL_PREBUILT += $(file)
+$(INSTALLED_RAMDISK_TARGET): $(file)
 endif
 
 # Just like /system/etc/init.goldfish.sh, the /init.godlfish.rc is here