fix(network): enable USE_SETUP_PRIORITY_OVERRIDE when priority is configured

PR #14255 generates calls to Component::set_setup_priority(float) from
ethernet/wifi to_code(), but that method's body in core/component.cpp is
gated by #ifdef USE_SETUP_PRIORITY_OVERRIDE. Without the define the
declaration exists but no implementation is linked, producing:

  undefined reference to `esphome::Component::set_setup_priority(float)`

The existing convention in cpp_helpers.register_component() is to add
the define whenever CONF_SETUP_PRIORITY appears in a component's YAML.
Mirror that here: when the user declares `network: priority:`, the
priority-driven setup_priority overrides will be emitted, so the define
must be on.

esphome/components/network/__init__.py

@@ -335,6 +335,10 @@ async def to_code(config):
     if CONF_PRIORITY in config:
         priority_list = config[CONF_PRIORITY]
         CORE.data[KEY_NETWORK_PRIORITY] = priority_list
+        # Enable Component::set_setup_priority() so the per-interface to_code
+        # calls below have a defined symbol to link against. Without this define
+        # the implementation in core/component.cpp is compiled out.
+        cg.add_define("USE_SETUP_PRIORITY_OVERRIDE")
 
         def _fmt(entry):
             if entry["timeout"] is not None: