The ornamental design for a built-in griddle, as shown and described.
1. Built-in griddle
1.1 is a front perspective view of a built-in griddle;
1.2 is a front view thereof;
1.3 is a back view thereof;
1.4 is a left view thereof where the right view is a mirror image;
1.5 is a top view thereof; and
1.6 is a bottom view thereof.
The dash-dash broken lines depict portions of the built-in griddle that form no part of the claimed design. The dash-dot broken lines define the boundary of the claimed design and form no part thereof.