Build: Fix missing Cmake command win_fix_dir_sep

Adds include of Utils.cmake to FindPlantUML.cmake. Solves the
building error `Unknown CMake command win_fix_dir_sep`.

Change-Id: Iaed5a6cc92e96832e668f32065d73d28d59033e5
Signed-off-by: Tudor Cretu <tudor.cretu@arm.com>
1 file changed