Fix clang-tidy redundant-declaration warning

Include hal.h directly in scheduler.h instead of forward-declaring
millis_64(), which clang-tidy flags as redundant when both headers
are included in the same translation unit.

esphome/core/scheduler.h

@@ -10,13 +10,11 @@
 #endif
 
 #include "esphome/core/component.h"
+#include "esphome/core/hal.h"
 #include "esphome/core/helpers.h"
 
 namespace esphome {
 
-// Declared in hal.h - forward declare to avoid circular include
-uint64_t millis_64();
-
 class Component;
 struct RetryArgs;