Make buildinfo_prop depend on build number file

buildinfo.prop should be rebuilt when the build number file changes,
because it needs to contain the up to date build number as well.

When this was implemented in make, that dependency was added. But when
we converted it to soong, it was removed. (aosp/3015794 and
aosp/3015818)

Fixes: 332733525
Test: m out/target/product/emu64x/obj/ETC/buildinfo.prop_intermediates/buildinfo.prop repeatedly, see that it rebuilds
Change-Id: Ife9bfa7d48f2578984761f0a78eda3513b324298