fix some function declarations

src/modbus-rtu.c

@@ -267,7 +267,7 @@ static void _modbus_rtu_ioctl_rts(modbus_t *ctx, int on)
 }
 #endif
 
-ssize_t _modbus_rtu_write_n_read(modbus_t *ctx, const uint8_t *req, int req_length) {
+static ssize_t _modbus_rtu_write_n_read(modbus_t *ctx, const uint8_t *req, int req_length) {
  ssize_t w, r, i;
  uint8_t rb[512];