drm_hwcomposer: Use pragma once instead of include guards

Some of existing include guards were copy-pasted without modification,
therefore have incorrect name. Switch to 'pragma once' in order to fix
it and avoid such copy-paste issues in the future.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/backend/BackendClient.h b/backend/BackendClient.h
index 95abb0f..1333eca 100644
--- a/backend/BackendClient.h
+++ b/backend/BackendClient.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_BACKEND_CLIENT_H
-#define ANDROID_BACKEND_CLIENT_H
+#pragma once
 
 #include "Backend.h"
 
@@ -27,5 +26,3 @@
                               uint32_t *num_requests) override;
 };
 }  // namespace android
-
-#endif