Add joystick plugin (#39)

https://github.com/sony/flutter-elinux-plugins/issues/35

packages/joystick/elinux/linux_joystick.cc

@@ -0,0 +1,27 @@
+// Copyright 2021 Sony Group Corporation. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/joystick.h>
+#include <stdio.h>
+#include <unistd.h>
+
+extern "C" __attribute__((visibility("default"))) int joystick_open(
+    const char* device) {
+  int fd = open(device, O_NONBLOCK);
+  if (fd < 0) {
+    fprintf(stderr, "Failed to open %s (%d)\n", device, errno);
+  }
+  return fd;
+}
+
+extern "C" __attribute__((visibility("default"))) int joystick_read(
+    int fd, js_event* ev) {
+  int bytes = read(fd, ev, sizeof(*ev));
+  if (bytes < 0) {
+    return -1;
+  }
+  return bytes == sizeof(*ev);
+}