[source] gre: add empty install rules for grev4 & grev6

[LEDE Commits]

jow pushed a commit to source.git, branch master:
https://git.lede-project.org/5ab258e57a5342160cf6c225c265828efd52cee4

commit 5ab258e57a5342160cf6c225c265828efd52cee4
Author: Alexandru Ardelean <ardeleanalex at gmail.com>
AuthorDate: Tue Nov 8 16:26:47 2016 +0200

    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 at gmail.com>
---
 package/network/config/gre/Makefile | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/package/network/config/gre/Makefile b/package/network/config/gre/Makefile
index 2e05ffc..4511620 100644
--- a/package/network/config/gre/Makefile
+++ b/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))