Enable test build of all public headers in lhcb-gaudi-head
The test build of public headers should be enabled on all slots (that support it), but I already know from a private test that some headers do not compile, so I temporarily enable it only in lhcb-guadi-head. Once the problems are fixed, this should be enabled in lhcb-master at least.
See lhcb/LHCb!3894 (comment 6477749)
/cc @rmatev