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/include/InputThread.h b/services/inputflinger/include/InputThread.h
index 407365a..5e75027 100644
--- a/services/inputflinger/include/InputThread.h
+++ b/services/inputflinger/include/InputThread.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _UI_INPUT_THREAD_H
-#define _UI_INPUT_THREAD_H
+#pragma once
 
 #include <utils/Thread.h>
 
@@ -42,5 +41,3 @@
 };
 
 } // namespace android
-
-#endif // _UI_INPUT_THREAD_H
\ No newline at end of file