The ornamental design for a strap, as shown and described.
FIG. 1 is a perspective view of a strap;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a front elevational view thereof.
The broken lines showing the strap are included for the purpose of showing environmental structure that forms no part of the claimed design. The broken lines showing the symbolic break in the strap indicates that no portion between the break lines forms any part of the claimed design.