Disable update_engine targets for non-linux hosts.

The dependencies of this Android.mk are not defined in mac, even for
the target build, so we can't build update_engine on mac.

Bug: 23084776
Test: `mma` on linux.

Change-Id: I0813d900932155ea5648a60d28350baf9f12771b
diff --git a/Android.mk b/Android.mk
index e3b4ae5..be9c761 100644
--- a/Android.mk
+++ b/Android.mk
@@ -14,6 +14,8 @@
 # limitations under the License.
 #
 
+ifeq ($(HOST_OS),linux)
+
 LOCAL_PATH := $(my-dir)
 
 # Definitions applying to all targets. $(eval) this last.
@@ -305,3 +307,5 @@
     dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xml
 LOCAL_DBUS_PROXY_PREFIX := update_engine
 include $(BUILD_STATIC_LIBRARY)
+
+endif # HOST_OS == linux