[gnome-build-meta] Use GitHub as source for exiv2

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 97caa50199b547823f82148cb211e420745dd0b6
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Mon May 28 09:10:51 2018 -0500

    Use GitHub as source for exiv2
    
    exiv2.org is down, it's too unreliable to be a source for us

 elements/core-deps/exiv2.bst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/elements/core-deps/exiv2.bst b/elements/core-deps/exiv2.bst
index 933b68b..b08938b 100644
--- a/elements/core-deps/exiv2.bst
+++ b/elements/core-deps/exiv2.bst
@@ -1,7 +1,7 @@
 kind: cmake
 sources:
 - kind: tar
-  url: http://www.exiv2.org/builds/exiv2-0.26-trunk.tar.gz
+  url: github_com:Exiv2/exiv2/archive/v0.26.tar.gz
 depends:
 - filename: base.bst
   junction: freedesktop-sdk.bst