[CMake] Update FindProtobuf.cmake

[Kirill A. Korinskiy]

Hi guys!

I have a project with typical structe:

.
|-- CMakeLists.txt
|-- proto
|   `-- Subscriber.proto
`-- src
    `-- main.cc

and standard FindProtobuf create a custom command for generated pb.cc and
pb.h in CMAKE_CURRENT_BINARY_DIR and don't work if I have a path
"proto/Subscriber.proto" in attaches file have a fix for this issue.

-- 
wbr, Kirill
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