gvariant-internal.h: fix the include hack

gvariant-internal.h was defining GLIB_COMPILATION so that it could
include individual headers, but this broke tests/gvariant on windows
because setting GLIB_COMPILATION changes the definition of GLIB_VAR,
causing external variables to not be found. Fix this by having it
define __GLIB_H_INSIDE__ instead.

https://bugzilla.gnome.org/show_bug.cgi?id=688109