The ornamental design for an "air filter grill", as shown and described.
FIG. 1 is a rear perspective view from the left side of the air filter
grill;
FIG. 2 is a front perspective view from the left side; and,
FIG. 3 is a top view.
The broken line showing of the air filter in all of the FIGURES is for
illustrative purposes only as to where the grill is positioned and forms
no part of the claimed design.