We claim the ornamental design for a locking packaging container, as shown and described.
FIG. 1 is a perspective view of a locking packaging container in an open configuration;
FIG. 2 is a right view thereof in an open configuration;
FIG. 3 is a left view thereof in an open configuration;
FIG. 4 is a rear view thereof in a closed configuration;
FIG. 5 is a front view thereof in a closed configuration;
FIG. 6 is a top view thereof in an open configuration; and,
FIG. 7 is a bottom view thereof in an open configuration. The broken lines depict portions of the locking packaging container that form no part of the claimed design.