libfuse: needs threads

libfuse needs threads, fixes:
http://autobuild.buildroot.net/results/0111a67b9ed5f5f5dfc0725d009ea5c756c1755c



[Peter: update sshfs/unionfs comments to match]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>