ActsTrkEvent: Fix compilation with clang15.

Missing template keywords.

This merge request affects 1 package:

• Tracking/Acts/ActsTrkEvent

This merge request affects 1 file:

• Tracking/Acts/ActsTrkEvent/ActsTrkEvent/MultiTrajectory.h

Adding @toyamaza ,@tbold ,@amorley ,@cvarni ,@pagessin as watchers

added ACTS Tracking master review-pending-level-1 labels

CI Result SUCCESS (hash 00a8b826)

	Athena
externals
cmake
make
tests

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Athena: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 63158]

added review-approved-tier0 label

removed review-pending-level-1 label

added review-approved label

merged

mentioned in commit 97b0c428

added sweep:ignore label