system/gatekeeper: use proper nativehelper headers

libnativeheader exports headers under nativeheader. These were
available before incorrectly as global headers in order to give
access to jni.h.

Test: modules using system/gatekeeper find headers
Bug: 63762847
Change-Id: I1c238bec06b23ddff0550f3abc7670839f432b1b
diff --git a/gatekeeper.cpp b/gatekeeper.cpp
index 44993cf..76fc8d6 100644
--- a/gatekeeper.cpp
+++ b/gatekeeper.cpp
@@ -13,8 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-#include <UniquePtr.h>
 #include <gatekeeper/gatekeeper.h>
+#include <nativehelper/UniquePtr.h>
 
 #include <endian.h>