gobject: Export gobject_init() function when doing static compilation

When statically compiling GObject and another GObject based library into the same shared object, the constructors need to be called in the right order.

This is the same as gobject_win32_init(), except that it's not specific to Windows. The same thing applies on other platforms.

My goal is to be able to link the glib-networking plugins into the same shared object as glib itself.