Fixes warnings when installing with OSX/homebrew

As shown below, fixes the following:

```bash
josh@kiva: Desktop/zklua> make
gcc -c `pkg-config --cflags lua5.1` -fPIC -O2  zklua.c
Package lua5.1 was not found in the pkg-config search path.
Perhaps you should add the directory containing `lua5.1.pc'
to the PKG_CONFIG_PATH environment variable
No package 'lua5.1' found
zklua.c:291:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
zklua.c:312:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
zklua.c:689:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
zklua.c:697:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
zklua.c:705:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
5 warnings generated.
```