(maint) Bump cpp-pcp-client submodule
(maint) Bump cpp-pcp-client submodule

Without this AppVeyor fails because the old cpp-pcp-client used SSH for
the Leatherman submodule. Bump to a version that uses HTTPS.

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