The ornamental design for a holdingbar for a road construction machine, as shown and described.
FIG. 1 is a perspective view of a holding bar for a road construction machine showing my new design;
FIG. 2 is a left elevation view thereof;
FIG. 3 is a front elevation view thereof
FIG. 4 is a right elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a top plan view thereof.
Dashed lines showing apertures in the holding bar for a road construction machine are included for the purpose of illustrating means for fastening the holding bar to the road construction machine, and form no part of the claimed design.