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/BackendManager.h b/backend/BackendManager.h
index 751cb78..4976744 100644
--- a/backend/BackendManager.h
+++ b/backend/BackendManager.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_BACKEND_MANAGER_H
-#define ANDROID_BACKEND_MANAGER_H
+#pragma once
 
 #include <functional>
 #include <map>
@@ -54,5 +53,3 @@
   std::map<std::string, BackendConstructorT> available_backends_;
 };
 }  // namespace android
-
-#endif