Use multi-line comments to fix GitHub Actions build issue

[jdeokkim]

https://github.com/sheredom/json.h/actions/runs/18078787637/job/51439079550

[  4%] Building CXX object CMakeFiles/json_test.dir/allocator.cpp.o
[  8%] Building CXX object CMakeFiles/json_test.dir/allow_c_style_comments.cpp.o
[ 12%] Building C object CMakeFiles/json_test.dir/allow_equals_in_object.c.o
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[ 20%] Building C object CMakeFiles/json_test.dir/allow_hexadecimal_numbers.c.o
[ 25%] Building C object CMakeFiles/json_test.dir/allow_inf_and_nan.c.o
[ 29%] Building C object CMakeFiles/json_test.dir/allow_json5.c.o
[ 33%] Building C object CMakeFiles/json_test.dir/allow_leading_or_trailing_decimal_point.c.o
[ 37%] Building C object CMakeFiles/json_test.dir/allow_leading_plus_sign.c.o
[ 41%] Building C object CMakeFiles/json_test.dir/allow_location_information.c.o
[ 45%] Building C object CMakeFiles/json_test.dir/allow_multi_line_strings.c.o
[ 50%] Building C object CMakeFiles/json_test.dir/allow_no_commas.c.o
[ 54%] Building C object CMakeFiles/json_test.dir/allow_simplified_json.c.o
[ 58%] Building C object CMakeFiles/json_test.dir/allow_single_quoted_strings.c.o
[ 62%] Building CXX object CMakeFiles/json_test.dir/allow_trailing_comma.cpp.o
[ 66%] Building C object CMakeFiles/json_test.dir/allow_unquoted_keys.c.o
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[ 75%] Building CXX object CMakeFiles/json_test.dir/main.cpp.o
[ 79%] Building C object CMakeFiles/json_test.dir/test.c.o
In file included from /home/runner/work/json.h/json.h/test/test.c:28:
/home/runner/work/json.h/json.h/test/../json.h:943:7: error: // comments are not allowed in this language [-Werror,-Wcomment]
  943 |       // recursion error
      |       ^
1 error generated.
gmake[2]: *** [CMakeFiles/json_test.dir/build.make:331: CMakeFiles/json_test.dir/test.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:87: CMakeFiles/json_test.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2