[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