Fix imgtool publishing missing python package

Add missing `packaging` to allow version comparison.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/ci/imgtool_install.sh b/ci/imgtool_install.sh
index 69b60f0..94264f6 100755
--- a/ci/imgtool_install.sh
+++ b/ci/imgtool_install.sh
@@ -17,5 +17,5 @@
     exit 0
 fi
 
-pip3 install setuptools twine
+pip3 install setuptools twine packaging
 pip3 install --pre imgtool