For other people interested in this, you can find the signature in the file
Runtime/Engine/Classes/Components/PrimitiveComponent.h
It looks like this:
DECLARE_DYNAMIC_MULTICAST_DELEGATE_FiveParams( FComponentHitSignature, UPrimitiveComponent*, HitComponent, AActor*, OtherActor, UPrimitiveComponent*, OtherComp, FVector, NormalImpulse, const FHitResult&, Hit );