I claim the ornamental design for a backup alarm, as shown and described.
FIG. 1 is a perspective view of the front of a backup alarm;
FIG. 2 is another perspective view of the front of the backup alarm; and,
FIG. 3 is a front plan view of the backup alarm.
The broken lines illustrate portions of the article which form no part of the claimed design. The external rear, left, right, top, and bottom sides of the backup alarm form no part of the claimed design.