inputflinger: Use #pragma once instead of explicit define guards

The #pragma once directive is commonly used by modern cpp code, and
support for its use in Andorid is proposed in go/droidcppstyle.

Bug: 245989146
Test: Build, Presubmit
Change-Id: Id257f056972b753f505afe1d253d306bb1824098
diff --git a/services/inputflinger/InputManager.h b/services/inputflinger/InputManager.h
index b40ab7f..1137193 100644
--- a/services/inputflinger/InputManager.h
+++ b/services/inputflinger/InputManager.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _UI_INPUT_MANAGER_H
-#define _UI_INPUT_MANAGER_H
+#pragma once
 
 /**
  * Native input manager.
@@ -130,5 +129,3 @@
 };
 
 } // namespace android
-
-#endif // _UI_INPUT_MANAGER_H