build: work around broken kernel header

I got this scary warning during ./configure on rawhide:

checking linux/if_bridge.h usability... no
checking linux/if_bridge.h presence... yes
configure: WARNING: linux/if_bridge.h: present but cannot be compiled
configure: WARNING: linux/if_bridge.h:     check for missing prerequisite headers?
configure: WARNING: linux/if_bridge.h: see the Autoconf documentation
configure: WARNING: linux/if_bridge.h:     section "Present But Cannot Be Compiled"
configure: WARNING: linux/if_bridge.h: proceeding with the compiler's result
configure: WARNING:     ## ------------------------------------- ##
configure: WARNING:     ## Report this to libvir-list@redhat.com ##
configure: WARNING:     ## ------------------------------------- ##
checking for linux/if_bridge.h... no

* configure.ac (AC_CHECK_HEADERS): Provide struct in6_addr, since
linux/if_bridge.h uses it without declaring it.
(cherry picked from commit c308a9ae)

configure.ac

@@ -846,9 +846,13 @@ fi
 dnl
 dnl check for kernel headers required by src/bridge.c
 dnl
-if test "$with_qemu" = "yes" || test "$with_lxc" = "yes" ; then
-  AC_CHECK_HEADERS([linux/param.h linux/sockios.h linux/if_bridge.h linux/if_tun.h],,
-                   AC_MSG_ERROR([You must install kernel-headers in order to compile libvirt with QEMU or LXC support]))
+if test "$with_linux" = "yes"; then
+  if test "$with_qemu" = "yes" || test "$with_lxc" = "yes" ; then
+    AC_CHECK_HEADERS([linux/param.h linux/sockios.h linux/if_bridge.h linux/if_tun.h],,
+      [AC_MSG_ERROR([You must install kernel-headers in order to compile libvirt with QEMU or LXC support])],
+      [[#include <netinet/ip6.h>
+      ]])
+  fi
 fi