make: protobuf -- Escape slashes to future needs

At moment we generate protobuf dependencies having in
mind that protobuf/ directory is _always_ in the source
code root. This may change one day in future so better to
be on safe side and escape slashes immediately.

I meet this problem when been needed to build protobuf
builtin in $(root)/src/protobuf directory.

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