Update prebuilt build tools path

Build tools have been moved from prebuilts/sdk/tools to
prebuilts/build-tools.

Bug: 27857138
Change-Id: I60be42c197d420897dab77808c25818c9d2e81df
diff --git a/core/ninja.mk b/core/ninja.mk
index f5d9c89..8a94f39 100644
--- a/core/ninja.mk
+++ b/core/ninja.mk
@@ -1,4 +1,4 @@
-NINJA ?= prebuilts/ninja/$(HOST_PREBUILT_TAG)/ninja
+NINJA ?= prebuilts/build-tools/$(HOST_PREBUILT_TAG)/bin/ninja
 
 include $(BUILD_SYSTEM)/soong.mk