fix compile warnning using cmake

[dota17]

While build using cmake locally, i got these warinnings:

[ 90%] Building CXX object src/test_lib_json/CMakeFiles/jsoncpp_test.dir/main.cpp.o
/data/jsoncpp/src/test_lib_json/main.cpp: In member function ‘virtual void TestCharReaderAllowDropNullTestissue178::runTestCase()’:
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
   };
   ^
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
/data/jsoncpp/src/test_lib_json/main.cpp:3324:3: warning: missing initializer for member ‘TestCharReaderAllowDropNullTestissue178::runTestCase()::TestSpec::onRoot’ [-Wmissing-field-initializers]
[100%] Linking CXX executable jsoncpp_test

this PR will fix them.

[coveralls]

Coverage remained the same at 93.699% when pulling ab4bd7b on dota17:fix-cmake-warn into 6bc55ec on open-source-parsers:master.