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/Backend.h b/backend/Backend.h
index 82b8fca..c24e818 100644
--- a/backend/Backend.h
+++ b/backend/Backend.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_BACKEND_H
-#define ANDROID_BACKEND_H
+#pragma once
 
 #include "hwc2_device/DrmHwcTwo.h"
 
@@ -41,5 +40,3 @@
       int client_start, size_t client_size);
 };
 }  // namespace android
-
-#endif