Add conditionalization for (potentially unused) struct and function (#1282) (#1348)

src/rp2_common/pico_async_context/async_context_threadsafe_background.c

diff --git a/src/rp2_common/pico_async_context/async_context_threadsafe_background.c b/src/rp2_common/pico_async_context/async_context_threadsafe_background.c
index 7890fe2..33f3d45 100644
--- a/src/rp2_common/pico_async_context/async_context_threadsafe_background.c
+++ b/src/rp2_common/pico_async_context/async_context_threadsafe_background.c
@@ -86,6 +86,7 @@
     }
 }
 
+#if ASYNC_CONTEXT_THREADSAFE_BACKGROUND_MULTI_CORE
 typedef struct sync_func_call{
     async_when_pending_worker_t worker;
     semaphore_t sem;
@@ -100,7 +101,7 @@
     sem_release(&call->sem);
     async_context_remove_when_pending_worker(context, worker);
 }
-
+#endif
 
 static void lock_release(async_context_threadsafe_background_t *self) {
     bool outermost = 1 == recursive_mutex_enter_count(&self->lock_mutex);