Fix RGBLINK failing to read args on certain machines

`char` has implementation-defined signedness, and if it's chosen to be unsigned,
then -1 gets converted to 255, which is then promoted back to `int` as... 255,
always failing the loop condition in src/link/main.c:118
`int8_t` has the correct signedness, but considering `musl_getopt_long_only`
returns `int`, better use that so as not to lose any bits

src/link/main.c

@@ -109,7 +109,7 @@ static void cleanup(void)
 
 int main(int argc, char *argv[])
 {
-	char optionChar;
+	int optionChar;
 	char *endptr; /* For error checking with `strtol` */
 	unsigned long value; /* For storing `strtoul`'s return value */