Fix -Wformat compiler warning in expr.c.

Android forces -Wformat on for all source.

  toys/pending/expr.c:116:28: warning: field precision should have type 'int',
                                       but argument has type 'long' [-Wformat]
      ret->s = xmprintf("%.*s", m[1].rm_eo-m[1].rm_so, target+m[1].rm_so);
                         ~~^~   ~~~~~~~~~~~~~~~~~~~~~