The ornamental design for a fastener for packaging, as shown and described.
1. Fastener for packaging
1.1 : Perspective
1.2 : Front
1.3: Top
1.4 : Left
1.5 : Left, unfolded open configuration
1.6 : Perspective, open configuration
Figure 1.1 represents a perspective view of the fastener for packaging; figure 1.2 represents a front view of the fastener; figure 1.3 represents a top view of the fastener; figure 1.4 represents a side view of the fastener; figure 1.5 represents a side view of the fastener in an open configuration; figure 1.6 represents a perspective view of the fastener in an open configuration.