Add void to function declarations with no args

SDCC 4.3.3 complains about the declaration not matching the prototype:

    error 283: function declarator with no prototype

Add `void` to the AVR functions as well, even though no warning is
issued by GCC.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/board/system76/addw3/gpio.c

@@ -34,7 +34,7 @@ struct Gpio __code WLAN_PWR_EN =    GPIO(D, 3);
 struct Gpio __code XLP_OUT =        GPIO(B, 4);
 // clang-format on
 
-void gpio_init() {
+void gpio_init(void) {
     // PWRSW WDT 2 Enable 2
     GCR9 = BIT(5);
     // PWRSW WDT 2 Enable 1