gre: add empty install rules for grev4 & grev6

Build seems to fail with:

```
Collected errors:
 * satisfy_dependencies_for: Cannot satisfy the following dependencies for X:
 * grev4 *
 * opkg_install_cmd: Cannot install package X
```

After adding an empty install rule, the failure goes away.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

package/network/config/gre/Makefile

@@ -60,6 +60,14 @@ define Package/gre/install
 	$(INSTALL_BIN) ./files/gre.sh $(1)/lib/netifd/proto/gre.sh
 endef
 
+define Package/grev4/install
+	:
+endef
+
+define Package/grev6/install
+	:
+endef
+
 $(eval $(call BuildPackage,gre))
 $(eval $(call BuildPackage,grev4))
 $(eval $(call BuildPackage,grev6))