libinput: 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: Ib7852f1a343864dff46e7bc616d707bc7d5bf512
diff --git a/include/input/VirtualKeyMap.h b/include/input/VirtualKeyMap.h
index 6e8e2c9..a4381ea 100644
--- a/include/input/VirtualKeyMap.h
+++ b/include/input/VirtualKeyMap.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _LIBINPUT_VIRTUAL_KEY_MAP_H
-#define _LIBINPUT_VIRTUAL_KEY_MAP_H
+#pragma once
 
 #include <stdint.h>
 
@@ -77,5 +76,3 @@
 };
 
 } // namespace android
-
-#endif // _LIBINPUT_KEY_CHARACTER_MAP_H