Restore gvc submodule

Commit 68f439425bd accidentally dropped the changes from the last
couple of months ...

diff --git a/src/gvc b/src/gvc
index 0a79019088ce2f50b671c7964b8fd69c24a465b9..25bf3ed75fa604fa73e8b25241119a993fc659d6 160000 (submodule)
--- a/src/gvc
+++ b/src/gvc
@@ -1 +1 @@
-Subproject commit 0a79019088ce2f50b671c7964b8fd69c24a465b9
+Subproject commit 25bf3ed75fa604fa73e8b25241119a993fc659d6