put fwd declaration of void_caster into its proper namespace; corrected fwd declaration of void_cast_register

[SVN r30210]

libs/serialization

@@ -1 +1 @@
-Subproject commit 0968503e63e55819da9d6492a776db3e4e0d1091
+Subproject commit 7063ba837e47d3508693c00aab8faed233572f53