BeamSpotMonitor: Suppress unused functions warnings

LHCb!4613 (merged) Adds some new methods to those generated by meta_enum. The beamspot monitor uses this privately, to generate a local enum, but does not use the new methods added in LHCb!4613 (merged). Because this private enum is local to the scope of the compilation unit, clang (correctly) concludes the new methods are unused and issues warnings. As we know these methods are not used in this case, and removing them is not trivial as they are generate by the meta_enum macro, the simplest solution is to turn off the warnings for this one compilation.