Remove return from void functions

[andrjohns]

Removes the return from a few void functions, as it was causing warnings under -Wpedantic:

  ./quickjs/quickjs.c:4573:5: warning: void function 'JS_FreeCString' should not return void expression [-Wpedantic]
  ./quickjs/quickjs.c:4578:5: warning: void function 'JS_FreeCStringRT' should not return void expression [-Wpedantic]
  ./quickjs/quickjs.c:4583:5: warning: void function 'JS_FreeCStringUTF16' should not return void expression [-Wpedantic]
  ./quickjs/quickjs.c:4588:5: warning: void function 'JS_FreeCStringRT_UTF16' should not return void expression [-Wpedantic]

[saghul]

See: #1404

[andrjohns]

See: #1404

Ack sorry for not checking properly!