uapi glibc compat: fix outer guard of net device flags enum

Fix a wrong condition preventing the higher net device flags
IFF_LOWER_UP etc to be defined if net/if.h is included before
linux/if.h.

The comment makes it clear the intention was to allow partial
definition with either parts.

This fixes compilation of userspace programs trying to use
IFF_LOWER_UP, IFF_DORMANT or IFF_ECHO.

Fixes: 4a91cb61

 ("uapi glibc compat: fix compile errors when glibc net/if.h included before linux/if.h")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>