allow utimes on mac os, only lutimes isn't supported

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
discard ErrNotSupportedPlatform

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6493 from vieux/fix_cp_mac_os

allow utimes on mac os, only lutimes isn't supported