Pedantically speaking, you want the following (note that Sin/x really has a discontinuity at x=0, but Sinc doesn't):
Integrate[Sinc[x]^2, {x, -Infinity, Infinity}]
(* Pi *)
This also works, but technically it's not the same:
Integrate[(Sin[x]/x)^2, {x, -Infinity, Infinity}]
(* Pi *)
