cmake: disable -fvar-tracking-assignments for ceph_dencoder.cc

cmake: disable -fvar-tracking-assignments for ceph_dencoder.cc

there are too many variables involved in main() of ceph_dencoder.cc, but
-fvar-tracking-assignments is enabled by default, when the compiler
finds that the number of variables exceeds the limit
"max-vartrack-size", it gives up, and tries again disabling this option.

this change avoids the recompilation of ceph_dencoder.cc and also silences
the warning of
```
/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/encoding/ceph_dencoder.cc:
In function ‘int main(int, const char**)’:
/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/encoding/ceph_dencoder.cc:289:5:
note: variable tracking size limit exceeded with
-fvar-tracking-assignments, retrying without
 int main(int argc, const char **argv)
     ^
```

Signed-off-by: Kefu Chai <kchai@redhat.com>