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/hwc2_device/DrmHwcTwo.h b/hwc2_device/DrmHwcTwo.h
index 2b8a74f..29f61da 100644
--- a/hwc2_device/DrmHwcTwo.h
+++ b/hwc2_device/DrmHwcTwo.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_DRM_HWC_TWO_H_
-#define ANDROID_DRM_HWC_TWO_H_
+#pragma once
 
 #include <hardware/hwcomposer2.h>
 
@@ -86,5 +85,3 @@
   uint32_t last_display_handle_ = kPrimaryDisplay;
 };
 }  // namespace android
-
-#endif