The ornamental design for a strut tubing, as shown and described.
FIG. 1 is a front perspective view of a strut tubing showing the new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a front view;
FIG. 4 is a right side view;
FIG. 5 is a rear view;
FIG. 6 is a top view; and,
FIG. 7 is a bottom view.
The dashed broken lines represent structure or features that form no part of the claimed design. The surface shading is intended to depict surface contour and forms no part of the claimed design.