[opencv3/opencv4] Check whether TARGET libprotobuf is defined

[MonicaLiu0311]

Fixes #36093, check if TARGET libprotobuf is defined before add_library (libprotobuf ...):

CMake Error at /opt/vcpkg/scripts/buildsystems/vcpkg.cmake:639 (_add_library):
  _add_library cannot create imported target "libprotobuf" because another
  target with the same name already exists.
Call Stack (most recent call first):
  /opt/vcpkg/vcpkg_installed/x64-linux/share/opencv3/OpenCVModules.cmake:22 (add_library)
  /opt/vcpkg/vcpkg_installed/x64-linux/share/opencv3/OpenCVConfig.cmake:133 (include)
  /opt/vcpkg/scripts/buildsystems/vcpkg.cmake:859 (_find_package)
  cmake/extra_modules.cmake:39 (find_package)
  cmake/extra_modules.cmake:162 (ov_generate_dev_package_config)
  CMakeLists.txt:142 (include)

• Changes comply with the maintainer guide
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