loader: Convert ext list helper functions to generic form

This allows future changes where the device and instance ext lists are different
types.
diff --git a/loader/trampoline.c b/loader/trampoline.c
index b4eca0f..304860a 100644
--- a/loader/trampoline.c
+++ b/loader/trampoline.c
@@ -106,7 +106,8 @@
         loader_delete_layer_properties(ptr_instance,
                                        &ptr_instance->instance_layer_list);
         loader_scanned_icd_clear(ptr_instance, &ptr_instance->icd_libs);
-        loader_destroy_ext_list(ptr_instance, &ptr_instance->ext_list);
+        loader_destroy_generic_list(ptr_instance, (struct loader_generic_list *)
+                                    &ptr_instance->ext_list);
         loader_platform_thread_unlock_mutex(&loader_lock);
         loader_heap_free(ptr_instance, ptr_instance);
         return res;
@@ -123,7 +124,7 @@
                                        &ptr_instance->instance_layer_list);
         loader_scanned_icd_clear(ptr_instance,
                                  &ptr_instance->icd_libs);
-        loader_destroy_ext_list(ptr_instance,
+        loader_destroy_generic_list(ptr_instance, (struct loader_generic_list *)
                                 &ptr_instance->ext_list);
         loader_platform_thread_unlock_mutex(&loader_lock);
         loader_heap_free(ptr_instance, ptr_instance);
@@ -144,7 +145,7 @@
                                        &ptr_instance->instance_layer_list);
         loader_scanned_icd_clear(ptr_instance,
                                  &ptr_instance->icd_libs);
-        loader_destroy_ext_list(ptr_instance,
+        loader_destroy_generic_list(ptr_instance, (struct loader_generic_list *)
                                 &ptr_instance->ext_list);
         loader.instances = ptr_instance->next;
         loader_platform_thread_unlock_mutex(&loader_lock);