Re-order win32.[ch] includes a little bit to unbreak build

Re-order includes to make `G_OS_WIN32` available and add `document.h` (instead of simply using forward declaration which wouldn't glue these two headers together) to make sure GeanyDocument is available.

@eht16, I haven't a chance to test on Win32 but according to your comments this should fix it. If you or anyone gets motivated, you could fix win32.c includes section to follow the new conventions added to HACKING file, otherwise just leave and next time I re-setup a Win32 VM for building, I'll do it.