As smc_diag.h is bundled, there is little sense to build AF_SMC support
conditionally.
* netlink_smc_diag.c [!AF_SMC]: Include xlat/addrfams.h under
XLAT_MACROS_ONLY, enable the rest of file.
* netlink_sock_diag.c: Include xlat/addrfams.h under XLAT_MACROS_ONLY.
(diag_decoders): Remove "ifdef AF_SMC" condition for [AF_SMC], reorder
in accordance with address family values.
* tests/nlattr_smc_diag_msg.c [!AF_SMC]: Define AF_SMC to 43, enable
the rest of test.