cmake: remove _FORTIFY_SOURCE define

cmake: remove _FORTIFY_SOURCE define

according to `dpkg-buildflags`, ubuntu 24 raised this value to
`-D_FORTIFY_SOURCE=3` which causes `error: "_FORTIFY_SOURCE" redefined`
compilation failures because Ceph itself adds `-D_FORTIFY_SOURCE=2`

`_FORTIFY_SOURCE` is a hardening option. both our rpm and debian builds
already specify that via environment variables, so Ceph's cmake should
leave it alone

Fixes: https://tracker.ceph.com/issues/72361
Signed-off-by: Casey Bodley <cbodley@redhat.com>