package/duma: add missing define when installing

Commit 3ab9774f (package/duma: do not let Makefile guess OS) added a
define to not let duma's build system guess the OS, mostly to avoid a
warning, but forgot to add that same define to the install commands.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>