Can't build with zig 15.1

```
zig build-exe src/zwmcmd.zig -lc -lX11 -lXft -I/usr/include/freetype2
src/zwmcmd.zig:11:9: error: local variable is never mutated
var arg_one = args.next() orelse return usage();
^~~~~~~
src/zwmcmd.zig:11:9: note: consider using 'const'
src/zwmcmd.zig:30:9: error: local variable is never mutated
var root = c.XDefaultRootWindow(display);
^~~~
src/zwmcmd.zig:30:9: note: consider using 'const'
src/zwmcmd.zig:28:9: error: local variable is never mutated
var display: *c.Display = c.XOpenDisplay(null) orelse return; // TODO Error
^~~~~~~
src/zwmcmd.zig:28:9: note: consider using 'const'
make: *** [makefile:4: zwmcmd] Error 1
```