kbuild: fix "mkdir -p" usage in scripts/package/mkspec

"mkdir -p" does not only mean not to complain if the directory already
exists, but also to create the parent directories if needed. This patch
removes "lib" from the list of directories to create as we will also create
"lib/modules".

Signed-off-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>