[core] Inline ControllerRegistry::register_controller into header

Tiny one-liner, no reason to keep it in the .cpp.

esphome/core/controller_registry.cpp

@@ -6,8 +6,6 @@ namespace esphome {
 
 StaticVector<Controller *, CONTROLLER_REGISTRY_MAX> ControllerRegistry::controllers;
 
-void ControllerRegistry::register_controller(Controller *controller) { controllers.push_back(controller); }
-
 }  // namespace esphome
 
 #endif  // USE_CONTROLLER_REGISTRY

esphome/core/controller_registry.h

@@ -34,7 +34,7 @@ class ControllerRegistry {
    * Controllers should call this in their setup() method.
    * Typically only APIServer and WebServer register.
    */
-  static void register_controller(Controller *controller);
+  static void register_controller(Controller *controller) { controllers.push_back(controller); }
 
 // Notify method declarations (generated from entity_types.inc)
 // NOLINTBEGIN(bugprone-macro-parentheses)