ceph.spec.in: add libatomic to BuildRequires on fedora

ceph.spec.in: add libatomic to BuildRequires on fedora

otherwise we'd have failures like

/opt/compiler-explorer/gcc-trunk-20220808/bin/../lib/gcc/x86_64-linux-gnu/13.0.0/../../../../x86_64-linux-gnu/bin/ld:
/tmp/ccVlMbVh.o: in function `std::atomic<tagged_ptr>::store(tagged_ptr,
std::memory_order)':
/opt/compiler-explorer/gcc-trunk-20220808/include/c++/13.0.0/atomic:273:
undefined reference to `__atomic_store_16'

when generating the building system using CMake on fedora 36.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>