uapi glibc compat: fix compilation when !__USE_MISC in glibc

These structures are defined only if __USE_MISC is set in glibc net/if.h
headers, ie when _BSD_SOURCE or _SVID_SOURCE are defined.

CC: Jan Engelhardt <jengelh@inai.de>
CC: Josh Boyer <jwboyer@fedoraproject.org>
CC: Stephen Hemminger <shemming@brocade.com>
CC: Waldemar Brodkorb <mail@waldemar-brodkorb.de>
CC: Gabriel Laskar <gabriel@lse.epita.fr>
CC: Mikko Rapeli <mikko.rapeli@iki.fi>
Fixes: 4a91cb61 ("uapi glibc compat: fix compile errors when glibc net/if.h included before linux/if.h")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>