build: update gnulib submodule to latest

* boostrap.conf (gnulib.modules): Add xvasprintf, which
had been omitted by mistake.
* src/copy.c, src/dd.c, src/test.c: Don't include verror.h,
as Gnulib removed it.

bootstrap.conf

@@ -310,6 +310,7 @@ gnulib_modules="
   xstrtol-error
   xstrtold
   xstrtoumax
+  xvasprintf
   year2038-recommended
   yesno
 "