neko: Fix build against glibc 2.26

The header file xlocale.h has been removed in glibc 2.26.

Quoting the release notes[1]:

* The nonstandard header <xlocale.h> has been removed. Most programs
  should use <locale.h> instead. If you have a specific need for the
  definition of locale_t with no other declarations, please contact
  libc-alpha@sourceware.org and explain.

I've backported HaxeFoundation/neko@e286c8f3301fa443255a3101d14b73924670
against version 2.1.0 and the build now succeeds.

Signed-off-by: aszlig <aszlig@nix.build>