Fix build while using -pedantic flag

[ismagilli]

With -pedantic flag I got following compiler warning:

../nob.h:434:67: warning: ISO C forbids empty initializer braces before C23 [-Wpedantic]
  434 | #define nob_cmd_run(cmd, ...) nob_cmd_run_opt((cmd), (Nob_Cmd_Opt){__VA_ARGS__})
      |                                                                   ^

It's even worse with -Werror flag.

This PR aims to fix this problem by adding default value for one of options (I chose async since it's first argument).