Fix compile failure with no-threads

The async code was causing a compile failure if no-threads was used.
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

crypto/async/arch/async_null.h

@@ -69,7 +69,7 @@ typedef struct async_fibre_st {
 # define async_set_ctx(nctx)                    0
 # define async_get_ctx()                        ((async_ctx *)NULL)
 # define async_fibre_swapcontext(o,n,r)         0
-# define async_fibre_makecontext(c)
+# define async_fibre_makecontext(c)             0
 # define async_fibre_free(f)
 # define async_fibre_init_dispatcher(f)
 # define async_get_pool()                       NULL