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ARROW-13273: [C++] Don't use .pc only in CMake paths for Requires.private

Because they can't be found by raw pkg-config usage.

Closes #10668 from kou/cpp-pc-in-cmake-path

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>