Update path to ninja

ninja has been moved from prebuilts/ninja to prebuilts/build-tools so it
can share the hermetic libc++.so there.

Change-Id: I0b6e76baaa6916a8c7fd19c684b792ccef2f9cb1
diff --git a/soong.bash b/soong.bash
index f695e62..6684681 100755
--- a/soong.bash
+++ b/soong.bash
@@ -40,4 +40,4 @@
     fi
 fi
 
-"prebuilts/ninja/${PREBUILTOS}/ninja" -f "${BUILDDIR}/build.ninja" -w dupbuild=err "$@"
+"prebuilts/build-tools/${PREBUILTOS}/bin/ninja" -f "${BUILDDIR}/build.ninja" -w dupbuild=err "$@"