[Vala] [bug] Fix support for constant and global symbol references

[pancake <pancake youterm com>]

Here's a patch adding support to handle constant and symbol references in global structures.

The thing is that without this patch, the compiler segfaults. There'r still situations
where the compiler segfaults, so maybe would be good to fix the segfault before pushing
this patch.

https://bugzilla.gnome.org/show_bug.cgi?id=618933

Can somebody review it? Thanks!

--pancake