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/addw1/gpio.c

@@ -44,7 +44,7 @@ struct Gpio __code WLAN_EN =        GPIO(J, 2);
 struct Gpio __code WLAN_PWR_EN =    GPIO(B, 0);
 // clang-format on
 
-void gpio_init() {
+void gpio_init(void) {
     // Enable LPC reset on GPD2
     GCR = 0x04;