Fix compilation warning.

xargs.c: In function 'fopen_cloexec_for_read_only':
xargs.c:387:1: warning: control reaches end of non-void function [-Wreturn-type]
* xargs/xargs.c (fopen_cloexec_for_read_only): Return result.

xargs/xargs.c

@@ -383,6 +383,7 @@ static FILE* fopen_cloexec_for_read_only (const char *file_name)
 	  errno = saved_errno;
 	  return NULL;
 	}
+      return result;
     }
 }