(maint) Add dependency on librt
(maint) Add dependency on librt

pxp-agent uses Boost.Thread directly, which on some platforms has a
dependency on librt. Previously it picked up the dependency as a
side-effect of using Leatherman, which is fragile when using Leatherman
shared libraries and Boost static libraries.

puppetlabs/pxp-agent,puppetlabs/pxp-agent,puppetlabs/pxp-agent