libbase: switch to #pragma once.

Bug: N/A
Test: builds
Change-Id: I125498ac4257ac4e901c6f2a5f1bfac1432d859d
diff --git a/include/android-base/strings.h b/include/android-base/strings.h
index 4d9fa34..9c35560 100644
--- a/include/android-base/strings.h
+++ b/include/android-base/strings.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef ANDROID_BASE_STRINGS_H
-#define ANDROID_BASE_STRINGS_H
+#pragma once
 
 #include <sstream>
 #include <string>
@@ -75,5 +74,3 @@
 
 }  // namespace base
 }  // namespace android
-
-#endif  // ANDROID_BASE_STRINGS_H