kamailio: add descriptions via BuildKamailioModule

The Makefile currently advertises that descriptions can be added by
specifying a string in argument 3 of BuildKamailioModule, but there's
actually no mechanism for this. This commit adds the missing pieces.
Signed-off-by: NSebastian Kemper <sebastian_ml@gmx.net>

net/kamailio/Makefile

@@ -370,6 +370,9 @@ define BuildKamailioModule
   endef
   define Package/kamailio-mod-$(subst _,-,$(1))/conffiles
 $(subst $(space),$(newline),$(foreach c,$(6),/etc/kamailio/$(c)))
+  endef
+  define Package/kamailio-mod-$(subst _,-,$(1))/description
+$(subst \n,$(newline),$(3))
   endef
   define Package/kamailio-mod-$(subst _,-,$(1))/install
 $(call Package/kamailio/install/module,$$(1),$(1))