imcforecast.m: fix faulty logic of warning

Steady state and time shift were missing

(cherry picked from commit 071ab1e1)
9 jobs for 4.6 in 351 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #16735
build_binaries

00:10:55

passed #16736
build_doc

00:01:13

 
  Test And Pkg
passed #16739
macOS
pkg_macOS

00:13:18

passed #16737
pkg_source

00:02:41

passed #16738
pkg_windows

00:23:46

passed #16743
test_dynare++

00:11:37

passed #16740
test_matlab

00:55:00

passed #16742
test_octave

04:49:53

passed #16741
test_old_matlab

01:11:46