chore: fix unused variable in ffi example (#1825)

# What does this PR do?

This PR fixes an unused variable (`c`) which happens when the code is compiled with `NDEBUG` (which I think I accidentally did when I ran FFI tests for #1733). To work around this, I extracted the `c.load` from the `assert` and then always do `std::ignore = value` so that everything is always used. 

Co-authored-by: thomas.kowalski <thomas.kowalski@datadoghq.com>

Author: KowalskiThomas

examples/ffi/array_queue.cpp

@@ -96,6 +96,8 @@ int main(void) {
   }
 
   for (const auto &c : counts) {
-    assert(c.load(std::memory_order_seq_cst) == num_threads);
+    auto value = c.load(std::memory_order_seq_cst);
+    assert(value == num_threads);
+    std::ignore = value;
   }
 }