[C++] Suppress sign-compare warning with g++ 9.2.1

In file included from ../src/arrow/status.h:24,
                 from ../src/arrow/memory_pool.h:26,
                 from ../src/arrow/buffer.h:28,
                 from ../src/arrow/array.h:28,
                 from ../src/arrow/array/builder_union.h:25,
                 from ../src/arrow/array/builder_union.cc:18:
../src/arrow/array/builder_union.cc: In constructor 'arrow::BasicUnionBuilder::BasicUnionBuilder(arrow::MemoryPool*, arrow::UnionMode::type, const std::vector<std::shared_ptr<arrow::ArrayBuilder> >&, const std::shared_ptr<arrow::DataType>&)':
../src/arrow/util/logging.h:86:55: error: comparison of integer expressions of different signedness: 'std::vector<arrow::ArrayBuilder*>::size_type' {aka 'long unsigned int'} and 'signed char' [-Werror=sign-compare]
   86 | #define ARROW_CHECK_LT(val1, val2) ARROW_CHECK((val1) < (val2))
      |                                                ~~~~~~~^~~~~~~~
../src/arrow/util/macros.h:43:52: note: in definition of macro 'ARROW_PREDICT_TRUE'
   43 | #define ARROW_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
      |                                                    ^
../src/arrow/util/logging.h:86:36: note: in expansion of macro 'ARROW_CHECK'
   86 | #define ARROW_CHECK_LT(val1, val2) ARROW_CHECK((val1) < (val2))
      |                                    ^~~~~~~~~~~
../src/arrow/util/logging.h:135:19: note: in expansion of macro 'ARROW_CHECK_LT'
  135 | #define DCHECK_LT ARROW_CHECK_LT
      |                   ^~~~~~~~~~~~~~
../src/arrow/array/builder_union.cc:63:3: note: in expansion of macro 'DCHECK_LT'
   63 |   DCHECK_LT(type_id_to_children_.size(), std::numeric_limits<int8_t>::max());
      |   ^~~~~~~~~