Fix some rp2040 hardware ID errors (#18617)

platforms/chibios/hardware_id.c

@@ -7,6 +7,9 @@
 hardware_id_t get_hardware_id(void) {
     hardware_id_t id = {0};
 #if defined(RP2040)
+    // Forward declare as including "hardware/flash.h" here causes more issues...
+    void flash_get_unique_id(uint8_t *);
+
     flash_get_unique_id((uint8_t *)&id);
 #elif defined(UID_BASE)
     id.data[0] = (uint32_t)(*((uint32_t *)UID_BASE));