Update fuzz.h

Author: Google-Autofuzz

src/test_lib_json/fuzz.h

@@ -9,6 +9,6 @@
 #include <cstddef>
 #include <stdint.h>
 
-int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size);
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size);
 
 #endif // ifndef FUZZ_H_INCLUDED