[unity-gtk-module] fix xinitrc.d-script breaking the GTK_MODULES environment variable

without this fix, the xinitrc.d-script takes everything thats in GTK_MODULES already,
makes it all uppercase and adds 'nity-gtk-module' straight after.
This is because of the ':u' immediately after $GTK_MODULES.
To prevent this, just use ${GTK_MODULES} to tell the shell exactly
what's the variable and what's the text to be appended

before, it made: 

    GTK_MODULES=canberra-gtk-module

to this:

    GTK_MODULES=CANBERRA-GTK-MODULEnity-gtk-module

which results in the following error message:

    Gtk-Message: Failed to load module "CANBERRA-GTK-MODULEnity-gtk-module"

(of course only, if you've got libcanberra installed, or some other gtk-module that gets added before the unity module)