This reverts commit 0773e51d92.
By the SLINT_COMPILER being a `FILEPATH`, cmake will convert any
relative path to an absolute path. That's not wanted for the Yocto
build, because there we pass
'$ENV{OECORE_NATIVE_SYSROOT}/usr/bin/slint-compiler', so that
the path is resolved dynamically against the environment variable that
the Yocto SDK sets.
CMake would convert this to an absolute path that points into the build
directory, i.e. it would be /home/blah/foo/\$ENV{...}