package/cpulimit: Added missing dependency on threads

cpulimit requires a toolchain supporting threads, see the following
build failure reports:

http://autobuild.buildroot.net/results/55e1af96f01a0786b4a95665736453db72be9c13
http://autobuild.buildroot.net/results/2e50face429553a6431ba5e8cda9527e5c876699



Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Fixes: 9e8e80d1 ("package: Add cpulimit")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>