Update libgnome-volume-control to master

Pick a few fixes, including a memory leak fix.

https://bugzilla.gnome.org/show_bug.cgi?id=695656

diff --git a/src/gvc b/src/gvc
index 03894efbcd010faf53bbc6e27256d27485f721b8..3d6aac673b88fff548db57a75b82e8f14723c1fe 160000 (submodule)
--- a/src/gvc
+++ b/src/gvc
@@ -1 +1 @@
-Subproject commit 03894efbcd010faf53bbc6e27256d27485f721b8
+Subproject commit 3d6aac673b88fff548db57a75b82e8f14723c1fe