ceph.spec.in: seastar drops _FORTIFY_SOURCE from CFLAGS also

ceph.spec.in: seastar drops _FORTIFY_SOURCE from CFLAGS also

the arrow submodule builds some C sources that trip up on _FORTIFY_SOURCE in debug builds

[ 79%] Building C object src/arrow/CMakeFiles/arrow_objlib.dir/vendored/musl/strptime.c.o
In file included from /usr/include/time.h:25,
                 from /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/17.0.0-10531-gc73e1fda/rpm/el8/BUILD/ceph-17.0.0-10531-gc73e1fda/src/arrow/cpp/src/arrow/vendored/strptime.h:20,
                 from /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/17.0.0-10531-gc73e1fda/rpm/el8/BUILD/ceph-17.0.0-10531-gc73e1fda/src/arrow/cpp/src/arrow/vendored/musl/strptime.c:4:
/usr/include/features.h:381:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
  381 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~
cc1: all warnings being treated as errors
make[5]: *** [src/arrow/CMakeFiles/arrow_objlib.dir/build.make:2543: src/arrow/CMakeFiles/arrow_objlib.dir/vendored/musl/strptime.c.o] Error 1

Signed-off-by: Casey Bodley <cbodley@redhat.com>