Add meta.branch to a bunch of packages

Gives better results in the monitor

pkgs/development/libraries/libsigcxx/1.2.nix

@@ -2,7 +2,7 @@
 
 stdenv.mkDerivation rec {
   name = "libsigc++-1.2.7";
-  
+
   src = fetchurl {
     url = "mirror://gnome/sources/libsigc++/1.2/${name}.tar.bz2";
     sha256 = "099224v5y0y1ggqrfc8vga8afr3nb93iicn7cj8xxgsrwa83s5nr";
@@ -13,5 +13,6 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = http://libsigc.sourceforge.net/;
     description = "A typesafe callback system for standard C++";
+    branch = "1.2";
   };
 }