Fix another Makefile typo.

Change-Id: I4a4a81255c3bef47c92c127e66c2ccd8b199e51d
diff --git a/build/Android.oat.mk b/build/Android.oat.mk
index 3272c27..592843e 100644
--- a/build/Android.oat.mk
+++ b/build/Android.oat.mk
@@ -57,7 +57,7 @@
   endif
   ifeq ($(1),optimizing)
     core_compile_options += --compiler-backend=Optimizing
-    core_dex2oat_dependency += $(DEX2OAT)
+    core_dex2oat_dependency := $(DEX2OAT)
     core_infix := -optimizing
   endif
   ifeq ($(1),interpreter)