exp-sgcheck/tests/hackedbz2.c mark vex_strlen as noinline

Explicitly set noinline so the test can check it is in the backtrace.
Newer gcc versions happily optimize it away otherwise.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14326 a5019735-40e9-0310-863c-91ae7b9d1cf9