forever-stack: Use rust_assert instead of assert

This causes complaints by our GCC 4.8 builder.

gcc/rust/ChangeLog:

	* resolve/rust-forever-stack.hxx: assert() -> rust_assert()

Author: CohenArthur

gcc/rust/resolve/rust-forever-stack.hxx

@@ -83,7 +83,7 @@ template <Namespace N>
 void
 ForeverStack<N>::pop ()
 {
-  assert (!cursor ().is_root ());
+  rust_assert (!cursor ().is_root ());
 
   rust_debug ("popping link");