Declare flash_default_config() in rtl837x_common.h

boot.c calls it from BANK3 and only rtlplayground.c had a prototype, so
the compiler saw an implicit declaration. The call itself was fine, the
function lives in the common area, but the warning was real. Move the
prototype into the shared header.

Reported by vDorst.

rtl837x_common.h

@@ -189,4 +189,5 @@ bool gpio_pin_test(uint8_t pin);
 void set_sys_led_state(uint8_t state);
 void sds_read(uint8_t sds_id, uint8_t page, uint8_t reg);
 void sds_write_v(uint8_t sds_id, uint8_t page, uint8_t reg, uint16_t v);
+void flash_default_config(void);
 #endif