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/DrmMode.h b/drm/DrmMode.h
index 20515f9..c6ddc63 100644
--- a/drm/DrmMode.h
+++ b/drm/DrmMode.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_DRM_MODE_H_
-#define ANDROID_DRM_MODE_H_
+#pragma once
 
 #include <xf86drmMode.h>
 
@@ -80,5 +79,3 @@
   std::string name_;
 };
 }  // namespace android
-
-#endif  // ANDROID_DRM_MODE_H_