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/drm/DrmDevice.h b/drm/DrmDevice.h
index 4cf0132..bb515e0 100644
--- a/drm/DrmDevice.h
+++ b/drm/DrmDevice.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_DRM_H_
-#define ANDROID_DRM_H_
+#pragma once
 
 #include <cstdint>
 #include <map>
@@ -24,7 +23,6 @@
 #include "DrmConnector.h"
 #include "DrmCrtc.h"
 #include "DrmEncoder.h"
-#include "DrmFbImporter.h"
 #include "utils/UniqueFd.h"
 
 namespace android {
@@ -121,5 +119,3 @@
   ResourceManager *const res_man_;
 };
 }  // namespace android
-
-#endif  // ANDROID_DRM_H_