ntdll: Work around futimens weak linking problem in set_file_times.

[Piotr Caban]

Newer XCode versions are weak linking to futimens function. Because of
that Wine will crash trying to execute the function on Mac OS <= 10.12.

Signed-off-by: Piotr Caban <piotr(a)codeweavers.com>
---
  dlls/ntdll/file.c | 26 +++++++++++++++++++++-----
  1 file changed, 21 insertions(+), 5 deletions(-)