FindProtobuf: Make outputs depend on protoc executable

After updating from Protocol Buffers 2.5.0 to 2.6.0 compilation of the
generated source failed: "This file was generated by an older version of
protoc which is incompatible with your Protocol Buffer headers. Please
regenerate this file with a newer version of protoc.".

Turns out the source and headers generated by way of
FindProtobuf.cmake:PROTOBUF_GENERATE_CPP aren't updated. Adding a
dependency on the compiler executable fixes this issue.
Merge topic 'FindProtobuf-depend-protoc' into next

e380d7c5 FindProtobuf: Make outputs depend on protoc executable