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.

bits/in.h

@@ -21,6 +21,9 @@
 # error "Never use <bits/in.h> directly; include <netinet/in.h> instead."
 #endif
 
+/* This is the generic version, do not assume a linux-based kernel.  */
+#define __USE_KERNEL_IPV6_DEFS 0
+
 /* To select the IP level.  */
 #define SOL_IP		0