Add missing dependency on soong.variables

I0dbc5ec58c89b869b59cd0602b82215c4972d799 dropped
configuration.ProductVariablesFileName from the ninja file dependencies.
This causes Soong not to rerun when soong.variables is modified.

Bug: 369141481
Test: m USE_HOST_MUSL=true nothing && m nothing, Soong reruns
Flag; EXEMPT bugfix

Change-Id: Ifb4c580eb72cad2f6555167fe40f43dcefa21d62