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

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

this silences the warning of

/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/osd/osdcap.cc:
In member function ‘virtual void
OSDCap_AllowClassMulti_Test::TestBody()’:
/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/osd/osdcap.cc:766:6:
note: variable tracking size limit exceeded with
-fvar-tracking-assignments, retrying without
 TEST(OSDCap, AllowClassMulti) {
      ^

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