pybind: use syntax supported by python3.6

pybind: use syntax supported by python3.6

Fixes: f68248944815acbf5fb08ef66f36da6d5baef824
It seems the shaman CentOS8 builders, which serve as the base for container images, aren't current with the ":=" syntax. Amend to support RHEL8, whose default python is python 3.6.8.

Error example from https://shaman.ceph.com/builds/ceph/ceph-nvmeof-mon/27188b78bdbaa3c33de518b8514906de629e774a/default/388913/

  File "/home/jenkins-build/build/workspace/ceph-dev-new-build/ARCH/x86_64/AVAILABLE_ARCH/x86_64/AVAILABLE_DIST/centos8/DIST/centos8/MACHINE_SIZE/gigantic/release/19.0.0-2505-g27188b78/rpm/el8/BUILD/ceph-19.0.0-2505-g27188b78/src/pybind/cephfs/setup.py", line 120
    if ldflags := os.environ.get('LDFLAGS'):
                ^
SyntaxError: invalid syntax

Signed-off-by: Alexander Indenbaum <aindenba@redhat.com>