There's no reason not to install these, and jit.h can be useful for
users of e.g. planner hooks.

Author: Donald Dong
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/296D405F-7F95-49F1-B565-389D6AA78505@csumb.edu
Backpatch: 11-, where JIT compilation was introduced