Adapt to new location of libcore

Change-Id: I17a2f432f69bd0cb381e83b6395fb2ae6e4faf9d
diff --git a/core/main.mk b/core/main.mk
index 0e53a98..dc81bbd 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -400,7 +400,7 @@
 subdirs += \
 	build/tools/signapk \
 	dalvik/dx \
-	dalvik/libcore \
+	libcore \
 	sdk/archquery \
 	sdk/androidprefs \
 	sdk/apkbuilder \