Rationalize visibility.

If it's C or POSIX, it's in. If it's BSD or GNU, it's guarded by __USE_BSD
or __USE_GNU.

Bug: https://code.google.com/p/android/issues/detail?id=194631
Change-Id: Ife51a21c2b37b060db56780d29c929805b199cb6
diff --git a/libc/include/android/set_abort_message.h b/libc/include/android/set_abort_message.h
index aebedba..18881a3 100644
--- a/libc/include/android/set_abort_message.h
+++ b/libc/include/android/set_abort_message.h
@@ -37,4 +37,4 @@
 
 __END_DECLS
 
-#endif // _SET_ABORT_MESSAGE_H
+#endif