qtbase: durable patch for Redox in6_pktinfo (not yet wired)

Records the fix that unblocked qtbase on Redox: relibc lacks struct in6_pktinfo,
which Qt sizes cmsg buffers with in both datagram paths. Define it at file scope
(outside the QT_NO_SCTP guard that Redox compiles out). The current qtbase.pkgar
was built from the equivalent in-tree source edit; this patch is the durable
form. NOT yet added to recipe.toml patches[] — wiring it invalidates the qtbase
cache and forces a full re-cook, so it is wired as the final step once the ISO
assembles (during the desktop-stack fix phase qtbase stays cached).

local/patches/qtbase/redox-in6-pktinfo.patch

@@ -0,0 +1,21 @@
+--- a/src/network/socket/qnativesocketengine_unix.cpp
++++ b/src/network/socket/qnativesocketengine_unix.cpp
+@@ -37,6 +37,18 @@
+ #  include <net/if_dl.h>
+ #endif
+ 
++#ifdef __redox__
++// Redox relibc does not define struct in6_pktinfo (IPV6_PKTINFO ancillary data),
++// but Qt sizes its cmsg control buffers with sizeof(struct in6_pktinfo) in both
++// the send and receive datagram paths. Define it at file scope (Linux layout) —
++// crucially OUTSIDE the QT_NO_SCTP guard above, which Redox compiles out.
++#include <netinet/in.h>
++struct in6_pktinfo {
++    struct in6_addr ipi6_addr;
++    unsigned int    ipi6_ifindex;
++};
++#endif
++
+ QT_BEGIN_NAMESPACE
+ 
+ /*