Search for and link to libsodium in standalone build
The LCG libzeromq is linked to libsodium, but pkg-config doesn't seem to propagate this, so explicitly search for and link to it. libsodium seems to be installed as a system library in all test systems, so this was not observed before.
LHCb!3423 (merged) takes care of the stack build, this MR is for the standalone build.