devel/ocl-icd: respect prefix after r435308

$ clinfo
Number of platforms 1

Platform Name                                   Intel Gen OCL Driver
Platform Vendor                                 Intel
Platform Version                                OpenCL 1.2 beignet 1.2

[...]
$ pkg upgrade -q
$ clinfo
Number of platforms 0
$ truss clinfo
[...]
open("/etc/OpenCL/vendors",O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC,022) ERR#2 'No such file or directory'
[...]

PR: 216739
Approved by: portmgr blanket