The ornamental design for a mainunit for a surveillance camera, as shown and described.
1. Main unit for a surveillance camera
1.1: Perspective
1.2: Top
1.3: Front
1.4: Left
1.5: Right
1.6: Rear view
1.7: Bottom
The dot-dash broken lines adjacent to the shaded areas represent the bounds of the claimed design. Portions of the main unit for a surveillance camera shown with broken lines form no part of the claimed design.