The ornamental design for a loader, as shown and described.
1. Loader
Figure 1.2 is a side view of a part of a loader showing our new design.
Figure 1.3 is a rear view of a part thereof.
Figure 1.4 is a rear perspective view thereof.
The broken lines showing parts of the tractor are included for the purpose of illustrating portions of the tractor and form no part of the claimed design.