package/python-psutil: add host variant

Add host variant to avoid the following build failure with mongodb since
drop of python3-psutil in commit
aab7bc4a:

make: *** No rule to make target 'host-python-psutil', needed by '/home/buildroot/autobuild/instance-3/output-1/build/mongodb-4.2.18/.stamp_configured'.  Stop.

Fixes:
 - http://autobuild.buildroot.org/results/3bce147b6b34d8854e54fc1102a047fcb2c19d01



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>