hostfs: fix mismatch between link_file definition and declaration

The function link_file declaration in the header file has the order
of the two arguments (from, to) swapped when compared to the definition
arguments of (to, from).  Fix this by swapping them around to match
the definition.

This error predates the git history, so no idea when this error
was introduced.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Richard Weinberger <richard@nod.at>