Use <fcntl.h> instead of <sys/fcntl.h>

When building on musl, we get:
```
In file included from tests/zfs-tests/cmd/getversion.c:22:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>

In file included from module/os/linux/zfs/vdev_file.c:36:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>                                                                                                                     |  ^~~~~~~                                                                                                                                                                         cc1: all warnings being treated as errors
```

Bug: https://bugs.gentoo.org/925235
Signed-off-by: Sam James <sam@gentoo.org>

module/os/linux/zfs/vdev_file.c

@@ -23,6 +23,7 @@
  * Copyright (c) 2011, 2020 by Delphix. All rights reserved.
  */
 
+#include <fcntl.h>
 #include <sys/zfs_context.h>
 #include <sys/spa.h>
 #include <sys/spa_impl.h>
@@ -33,7 +34,6 @@
 #include <sys/fs/zfs.h>
 #include <sys/fm/fs/zfs.h>
 #include <sys/abd.h>
-#include <sys/fcntl.h>
 #include <sys/vnode.h>
 #include <sys/zfs_file.h>
 #ifdef _KERNEL

tests/zfs-tests/cmd/getversion.c

@@ -19,9 +19,9 @@
  */
 
 #include <sys/ioctl.h>
-#include <sys/fcntl.h>
 #include <linux/fs.h>
 #include <err.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>