HPCC-11281 Fix OSX compile issues with file caching

Should fix OSX compile issues.
Or I could add to platform.h generic logic to determine if posix_fadvise() exists from XOPEN_SOURCE and/or POSIX_C_SOURCE values and use that macro.  Which is more preferable ?
Signed-off-by: Mark Kelly <mark.kelly@lexisnexis.com>