Update rt_linux.c

I met an error when compiling rt_linux.c with gcc 4.9.1 on Debian jessie/sid i386. Then I found a solution in <a  href="http://blog.sina.com.cn/s/blog_4d31f1650101ejlt.html">this page</a>.I think this change will not influence compiling on platforms that don't have this problem.
I will find out why the return type of function current_fsuid/current_fsgid is different on different platforms later.