correct rl_startup_hook type while USE_SYSTEM_READLINE=1

I set `USE_SYSTEM_READLINE=1` in my Make.user, and then Julia fails to build:
```
    PERL base/build_h.jl.phony
    CC ui/repl-readline.o
repl-readline.c:838:24: error: use of undeclared identifier 'Function'
    rl_startup_hook = (Function*)init_rl;
                       ^
repl-readline.c:838:33: error: expected expression
    rl_startup_hook = (Function*)init_rl;
                                ^
2 errors generated.
make[2]: *** [repl-readline.o] Error 1
make[1]: *** [julia-release] Error 2
make: *** [release] Error 2
```
I found there's no `Function` typedef in the system provided libreadline headers, so I changed it to `rl_hook_func_t`.