Define __USE_KERNEL_IPV6_DEFS macro for non-Linux kernels

Define __USE_KERNEL_IPV6_DEFS macro for non-Linux kernels

Commit 1c1e7fb6 changed the __USE_KERNEL_IPV6_DEFS tests from 'ifdef'
to 'if'. As inet/netinet.in.h is a generic file, this causes a warning
on non-Linux kernels (for example Hurd). To fix that define it in the
generic bits/in.h file.

Changelog:
	* bits/in.h (__USE_KERNEL_IPV6_DEFS): Define to 0.