Add header guard to mapping.h

Test: compile goog/master
Change-Id: I88ace1624463e79be416cb0c7e9c7f7d6fd20f0c
diff --git a/libhidlmemory/include/hidlmemory/mapping.h b/libhidlmemory/include/hidlmemory/mapping.h
index e15eb80..352cbaf 100644
--- a/libhidlmemory/include/hidlmemory/mapping.h
+++ b/libhidlmemory/include/hidlmemory/mapping.h
@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#ifndef ANDROID_HARDWARE_MAPPING_H
+#define ANDROID_HARDWARE_MAPPING_H
 #include <android/hidl/memory/1.0/IMemory.h>
 #include <hidl/HidlSupport.h>
 
@@ -30,3 +32,4 @@
 
 }  // namespace hardware
 }  // namespace android
+#endif