Fix build: __unused isn't unused!

Change-Id: I57f2b755d1b2509f9c5504b12a5ff6c50b78b375
diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index b483c16..f29381b 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -86,7 +86,7 @@
   struct ucontext *uc_link;
   stack_t uc_stack;
   sigset_t uc_sigmask;
-  char __unused[128 - sizeof(sigset_t)];
+  char __padding[128 - sizeof(sigset_t)];
   mcontext_t uc_mcontext;
 } ucontext_t;