COMP: Add missing call to cmake_minimum_required

The VXL_PIC_COMPATIBLE check CMakeLists.txt file recently started using
the cmake_policy() command.  This command is advanced enough to trigger
the CMP0000 requirement for cmake_minimum_required to be called.  Add
the missing call.

Change-Id: Ibfe630677604885c45ce61260ea30ed128e2c30c
Merge topic 'fix-CMP0000-warning'

82a381b0 COMP: Add missing call to cmake_minimum_required