netbsd.i18n_module: fix build

Tested building pkgsCross.x86_64-netbsd.netbsd.i18n_module from
x86_64-linux.

pkgs/os-specific/bsd/netbsd/default.nix

···
     version = "8.0";
     sha256 = "0w6y5v3binm7gf2kn7y9jja8k18rhnyl55cvvfnfipjqdxvxd9jd";
     meta.platforms = lib.platforms.netbsd;
+    extraPaths = [ libc.src ];
   };
 
   csu = mkDerivation {