Add task for API diffs

Test: make api-diff
Change-Id: Ifb788069bf1d882a52e8429e056070675f866986
diff --git a/core/config.mk b/core/config.mk
index 80f7487..c17b0f5 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -112,6 +112,7 @@
 BUILD_STATIC_JAVA_LIBRARY:= $(BUILD_SYSTEM)/static_java_library.mk
 BUILD_HOST_JAVA_LIBRARY:= $(BUILD_SYSTEM)/host_java_library.mk
 BUILD_DROIDDOC:= $(BUILD_SYSTEM)/droiddoc.mk
+BUILD_APIDIFF:= $(BUILD_SYSTEM)/apidiff.mk
 BUILD_COPY_HEADERS := $(BUILD_SYSTEM)/copy_headers.mk
 BUILD_NATIVE_TEST := $(BUILD_SYSTEM)/native_test.mk
 BUILD_NATIVE_BENCHMARK := $(BUILD_SYSTEM)/native_benchmark.mk