[esp8266] Add bare arch_get_cpu_cycle_count() decl in hal_esp8266.h

Followup #16111 dropped the cross-platform arch_get_cpu_cycle_count()
declaration from hal.h dispatcher and added bare decls to libretiny,
rp2040, host, and zephyr per-platform headers — but missed
hal_esp8266.h. ESP8266 callers (spi.h, uart_component_esp8266.cpp) now
fail clang-tidy with 'use of undeclared identifier'.

The body is still out-of-line in components/esp8266/core.cpp on this
PR; the inline def is added on the chained #16112 PR. Add a bare decl
here for now so #16111 builds in isolation.

esphome/core/hal/hal_esp8266.h

@@ -60,6 +60,8 @@ __attribute__((always_inline)) inline uint16_t progmem_read_uint16(const uint16_
 __attribute__((always_inline)) inline void delayMicroseconds(uint32_t us) { delay_microseconds_safe(us); }
 __attribute__((always_inline)) inline void arch_feed_wdt() { system_soft_wdt_feed(); }
 
+uint32_t arch_get_cpu_cycle_count();
+
 }  // namespace esphome
 
 #endif  // USE_ESP8266