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ARROW-8280: [C++] Use c-ares_INCLUDE_DIR

Instead of c-ares::cares's INTERFACE_INCLUDE_DIRECTORIES. Because it
may not be set. c-ares_INCLUDE_DIR is always set.

Closes #6778 from kou/ci-mingw-fix-cares

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