mountpoint-util: tmpfs supports uid=/gid= mount options

As tmpfs(5) says, both uid= and gid= are supported since kernel 2.5.7 and
the mount utility seems to agree:

```
# stat -c "%U:%G" mnt
root:root
# mount -o uid=testuser,gid=testuser -t tmpfs tmpfs mnt
# stat -c "%U:%G" mnt
testuser:testuser
```

However, systemd-mount currently complains:

```
# systemd-mount --owner testuser -t tmpfs tmpfs mnt
File system type tmpfs is not known to support uid=/gid=, refusing.
```

src/basic/mountpoint-util.c

@@ -480,6 +480,7 @@ bool fstype_can_uid_gid(const char *fstype) {
                           "iso9660",
                           "msdos",
                           "ntfs",
+                          "tmpfs",
                           "vfat");
 }