libc: remove C++ comments from public headers.

Change-Id: I4af84f912062cd2ff34711c25122fb323f20c032
diff --git a/libc/include/utmp.h b/libc/include/utmp.h
index e362b40..ffd3c92 100644
--- a/libc/include/utmp.h
+++ b/libc/include/utmp.h
@@ -87,4 +87,4 @@
 
 __END_DECLS
 
-#endif // _UTMP_H_
+#endif /* _UTMP_H_ */