Introduce support for attribute type bitfield32.
Note that since the generated code works with struct nla_bitfield32,
the generator adds netlink.h to the list of includes for userspace
headers in case any bitfield32 is present.

Note that this is added only to genetlink-legacy scheme as requested
by Jakub Kicinski.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20231021112711.660606-3-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>