The ornamental design for a headstrap, as shown and described.
FIG. 1 is a perspective view of a head strap showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a perspective view of the head strap where the head strap is in a configuration of use.
The broken lines in the FIGS. 1-8 depict portions of the head strap that form no part of the claimed design. The other broken lines in the FIG. 9 represent environment subject matter only and form no part of the claimed design.