package/duma: add missing define when installing

Commit 3ab9774 (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>

package/duma/duma.mk

@@ -25,11 +25,11 @@ define DUMA_BUILD_CMDS
 endef
 
 define DUMA_INSTALL_STAGING_CMDS
-	$(MAKE) prefix=$(STAGING_DIR)/usr install -C $(@D)
+	$(MAKE) OS=linux prefix=$(STAGING_DIR)/usr install -C $(@D)
 endef
 
 define DUMA_INSTALL_TARGET_CMDS
-	$(MAKE) prefix=$(TARGET_DIR)/usr install -C $(@D)
+	$(MAKE) OS=linux prefix=$(TARGET_DIR)/usr install -C $(@D)
 endef
 
 $(eval $(generic-package))