[esp8266] Address Copilot review on #16112

- hal.cpp: include esphome/core/helpers.h for the HOT macro (millis/delay
  use it; without the include the TU fails to compile).
- hal_esp8266.h: drop the redundant arch_get_cpu_cycle_count() bare
  declaration that came back via the merge from dev (the inline def
  above already declares the function).

esphome/components/esp8266/hal.cpp

@@ -1,6 +1,7 @@
 #ifdef USE_ESP8266
 
 #include "esphome/core/hal.h"
+#include "esphome/core/helpers.h"
 
 #include <Arduino.h>
 #include <core_esp8266_features.h>

esphome/core/hal/hal_esp8266.h

@@ -66,8 +66,6 @@ __attribute__((always_inline)) inline void arch_init() {}
 __attribute__((always_inline)) inline uint32_t arch_get_cpu_cycle_count() { return esp_get_cycle_count(); }
 __attribute__((always_inline)) inline uint32_t arch_get_cpu_freq_hz() { return F_CPU; }
 
-uint32_t arch_get_cpu_cycle_count();
-
 }  // namespace esphome
 
 #endif  // USE_ESP8266