add missing header guard to invoke.h

• [2017-10-24 11:58] Validation started with lhcb-gaudi-merge#255
• [2017-10-25 00:02] Validation started with lhcb-future-clang#353
• [2017-10-25 00:02] Validation started with lhcb-future#577

approved this merge request

changed milestone to %v30r0

assigned to @hegner

mentioned in merge request lhcb/LHCb!925 (merged)

merged

mentioned in commit 083d6417

added C++ framework label

mentioned in commit 5fe6c9d5

mentioned in merge request !459 (merged)

removed backport to v29 label

mentioned in commit ae1b4ff7