Fix standalone build of headers
With lhcb-core/lcg-toolchains!152 (merged) we always run a stand alone build of public headers to find possible errors.
This fixes the build of headers for the run2-patches branch.
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With lhcb-core/lcg-toolchains!152 (merged) we always run a stand alone build of public headers to find possible errors.
This fixes the build of headers for the run2-patches branch.