The ornamental design for neck prop, as shown and described.
1. Neck prop
1.1 : Back
1.2 : Perspective
1.3 : Right
1.4 : Bottom
1.5 : Front
1.6 : Left
1.7 : Top
The present design prevents the neck from moving when travelling by airplanes or public transport, and will prevent the shaking by supporting the neck, the lower part of the design has the protruding shape so that it can be worn comfortably.