keystone: set CMake policy version to fix build with newer CMake

[striezel]

Otherwise build fails with messages like

CMake Error at CMakeLists.txt:4 (cmake_minimum_required):
   Compatibility with CMake < 3.5 has been removed from CMake.

   Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
   to tell CMake that the project requires at least <min> but has been updated
   to work with policies introduced by <max> or earlier.

   Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

[MehdiChinoune]

keystone-engine/keystone#593 include some fixes.

[striezel]

Thanks.
Only updating the CMake policy was just a quick shot anyway. Let's see whether the CMake-related commit from that upstream pull request will suffice.

Edit: Seems to build properly now.