MuonTruthAlgs: cmake fixes

Define an interface library for exported headers.

This merge request affects 1 package:

• MuonSpectrometer/MuonTruthAlgs

Adding @rosati ,@nkoehler ,@wleight as watchers

added MuonSpectrometer master review-pending-level-1 labels

CI Result SUCCESS (hash 4ab427fc)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 19179]

added 21.1 review-approved review-pending-level-2 labels

removed review-pending-level-1 review-pending-level-2 labels

removed 21.1 label

Changes look fine, L1 approving.

merged

mentioned in commit 17eeec9a

added sweep:ignore label