The ornamental design for an endcap for an awning, as shown and described.
FIG. 1 is a bottom perspective view of an end cap for an awning;
FIG. 2 is a bottom view of an end cap for an awning;
FIG. 3 is a top view of an end cap for an awning;
FIG. 4 is a front end view of an awning end cap;
FIG. 5 is a side view of an end cap for an awning;
FIG. 6 is a rear end view of an end cap for an awning;
FIG. 7 is a perspective view of an end cap for an awning; and,
FIG. 8 is a perspective view of an end cap for an awning as attached to an awning element, with the broken line for illustrative purposes only and forming no part of the claimed design.