runtime/cgo: use correct lean and mean macro

WIN64_LEAN_AND_MEAN is not the correct macro to use and doesn't ever
exist.

Change-Id: I32a5523cc0f7cc3f3a4d022071cf81f88db39aa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/291634


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>