[llcpp] [build] Change data_deps to deps

Apparently using data_deps in compiled_action will not trigger a
re-execution of the tool when fidlgen_llcpp changes. Out of these
following potential locations for the fidlgen dependency I tried:

- in data_deps in executable("fidlgen_llcpp_zircon")
- in deps in executable("fidlgen_llcpp_zircon")
- in data_deps in the compiled_action
- in deps in the compiled_action

Only the last one will correctly trigger the execution of
fidlgen_llcpp_zircon.

Also need to put fidlgen_llcpp in the depfile of the update/validate
actions, to indicate dependency on incremental builds.

TEST: Edit a .go source file in fidlgen_llcpp and observe that
fx build -k 0 tools/fidlgen_llcpp_zircon:update is correctly run and
modifies the checked in bindings.

Change-Id: Ie5779bc67bcc88ac1fea2fa599faea36d3a3ce7b