Fix CMake warning related to FLANN_ROOT

[mvieth]

Fix #6094
I also tested with the Windows all-in-one installer, where everything seems to work fine as well.

[ijnek]

@mvieth @larshg Is there any chance this change can be put into the debian releases on Ubuntu Jazzy? Currently, any project that depends directly or transitively on PCL, gets this error unless developer warnings are disabled across their whole project with -Wno-dev, which isn't ideal.

[mvieth]

@ijnek We are not responsible for the apt packages, so that is not in our hands. You would have to contact the maintainers of the libpcl-dev apt package. However, from what I know, packages in already released Ubuntu distributions are not touched any more (unless maybe it is something really severe, like a security issue).

[ijnek]

Thanks @mvieth, I'll request the maintainers of libpcl-dev apt package. For the record - https://bugs.launchpad.net/ubuntu/+source/pcl/+bug/2111295