make: protobuf -- Fix deps generation on generated sources

There are a few nits in protobuf generation (which in most
cases are harmless but somethime may cause building procedure
to fail)

 - Deps files over generated sources should be produced only
   when _all_ .proto files are handled, because we include headers
   which are autogenerated as well, thus simply wait until everything
   is complete then use compiler to generate deps files

 - typo in non-clean targes, i rather should use proto-obj-y objects

Reported-by: Andrey Vagin <avagin@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>