The ornamental design for a warningsign, as shown and described.
5. Warning sign
5.1: Front
5.2: Back
5.3: Top
5.4: Right
The left view is omitted because it is identical with the right view; the bottom view is omitted because it is identical with the top view; the dot-dash broken lines define the boundaries of the claimed design and form no part of the claimed design; the broken lines inside the boundaries form part of the claimed design; any other broken lines illustrate portions of the warning sign and form no part of the claimed design.