signal.c: use mcontext_t

* signal.c (check_stack_overflow): use mcontext_t.  the struct tag
  name is not portable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45997 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

signal.c

@@ -710,7 +710,7 @@ NORETURN(void ruby_thread_stack_overflow(rb_thread_t *th));
 static void
 check_stack_overflow(const uintptr_t addr, const ucontext_t *ctx)
 {
-    const struct mcontext *mctx = ctx->uc_mcontext;
+    const mcontext_t *mctx = &ctx->uc_mcontext;
 # if defined __linux__
 #   if defined REG_RSP
     const greg_t sp = mctx->gregs[REG_RSP];