use microhttpd MHD_Result return type. fix: error: invalid conversion from int to MHD_Result

[milahu]

based on
cinemast/libjson-rpc-cpp#299

fix this build error

http_server.cc: In function 'MHD_Daemon* startupHttpDaemon(int)':
http_server.cc:722:31: error: invalid conversion from 'int (*)(void*, MHD_Connection*, const char*, const char*, const char*, const char*, size_t*, void**)' {aka 'int (*)(void*, MHD_Connection*, const char*, const char*, const char*, const char*, long unsigned int*, void**)'} to 'MHD_AccessHandlerCallback' {aka 'MHD_Result (*)(void*, MHD_Connection*, const char*, const char*, const char*, const char*, long unsigned int*, void**)'} [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-fpermissive-fpermissive8;;]
  722 |                         NULL, &answerToConnection, NULL, MHD_OPTION_CONNECTION_TIMEOUT, 5, MHD_OPTION_END);
      |                               ^~~~~~~~~~~~~~~~~~~
      |                               |
      |                               int (*)(void*, MHD_Connection*, const char*, const char*, const char*, const char*, size_t*, void**) {aka int (*)(void*, MHD_Connection*, const char*, const char*, const char*, const char*, long unsigned int*, void**)}