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/arch/avr/uart.c

@@ -42,7 +42,7 @@
 #endif
 // clang-format on
 
-int16_t uart_count() {
+int16_t uart_count(void) {
     return sizeof(UARTS) / sizeof(struct Uart);
 }