The ornamental design for a hacksaw, as shown and described.
FIG. 1 is a left/rear perspective view of a hacksaw.
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines showing the hacksaw blade is included for the purpose of illustrating environmental structure that forms no part of the claimed design.