Add dependency to /vendor/etc/linker.config.pb

The rule for the file should depend on vendor files. Otherwise, the rule
can be fired before installation of those stub libraries in /vendor/lib.

Bug: 244531518
Test: m (with a vendor lib with stubs)
Test: check /vendor/etc/linker.config.pb (should have the lib)
    $ conv_linker_config print -s $OUT/vendor/etc/linker.config.pb
Change-Id: If7a81e26a7a105234201b10cd58825c67a1d5967