Use -std=gnu89 in tools/c-index-test/CMakeLists.txt

With the old use of -std=c89 off_t is not defined and the build fails.

This seems to be another variation of
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40278.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198748 91177308-0d34-0410-b5e6-96231b3b80d8