jnr.posix.POSIX.fstat(int, FileStat) fails on Windows

[jfrantzius]

The following test unfortunately fails on Windows:

    @Test
    public void filestatDescriptor2() throws Throwable {
        File f = File.createTempFile("stat", null);

        try {
            int fd = posix.open(f.getAbsolutePath(), 0, 438);
            FileStat stat = posix.allocateStat();
            int result = posix.fstat(fd, stat);
            assertEquals(0, result);
            assertEquals(0, stat.st_size());
        } finally {
            f.delete();
        }

    }

With the following exception:

java.lang.IllegalArgumentException: instance of class jnr.posix.WindowsRawFileStat is not a struct
	at jnr.posix.BaseNativePOSIX$2.toNative(BaseNativePOSIX.java:746)
	at jnr.posix.BaseNativePOSIX$2.toNative(BaseNativePOSIX.java:1)
	at jnr.posix.WindowsLibC$jnr$ffi$0.fstat(Unknown Source)
	at jnr.posix.BaseNativePOSIX.fstat(BaseNativePOSIX.java:136)
	at jnr.posix.CheckedPOSIX.fstat(CheckedPOSIX.java:129)
	at jnr.posix.LazyPOSIX.fstat(LazyPOSIX.java:126)
[..]

The call to jnr.posix.POSIX.fstat(int, FileStat) currently isn't covered in jnr.posix.FileStatTest, which is why this (likely) bug may have gone unnoticed yet.