CMake: silence policy CMP0117 warning

- Warns when a Destination path is different from its normalized version
- This happens for the wheels' destination directory
- But this is benign as there's a trailing path separator that gets
  removed by the path normalization algorithm resulting in essentially
  the same original path

Change-Id: Ie6f6382941ea533d3b68b5acf275a0f7c07291d5
Signed-off-by: Mauricio Briceno <mauricio.briceno@arm.com>