added include for unistd.h that was missing

Before BOARD_VNDK_VERSION is defined, unistd.h was included via
utils/Log.h and then log/log.h in liblog.
This error is detected because include for unistd.h is missing for
log/log.h for include_vndk in liblog

Bug: 33241851
Test: built libutils successfully
Change-Id: Icf14f8a9dee46752536a57f707c4d8895496fd72
Merged-In: Ide4aee3cacad2a2bf6bd0cbdf0254c6c245f4020