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/HwcDisplay.h b/hwc2_device/HwcDisplay.h
index d79efb0..40c90a3 100644
--- a/hwc2_device/HwcDisplay.h
+++ b/hwc2_device/HwcDisplay.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_HWC2_DEVICE_HWC_DISPLAY_H
-#define ANDROID_HWC2_DEVICE_HWC_DISPLAY_H
+#pragma once
 
 #include <hardware/hwcomposer2.h>
 
@@ -239,5 +238,3 @@
 };
 
 }  // namespace android
-
-#endif