package/libnetfilter_conntrack: fix musl build with headers >= 5.15

Fix the following build failure with musl and headers >= 5.15 raised
since bump to version 1.0.9 in commit
9af9dfa5:

In file included from /home/buildroot/autobuild/instance-3/output-1/host/arm-buildroot-linux-musleabihf/sysroot/usr/include/arpa/inet.h:9,
                 from ../../include/internal/internal.h:14,
                 from proto.c:2:
/home/buildroot/autobuild/instance-3/output-1/host/arm-buildroot-linux-musleabihf/sysroot/usr/include/netinet/in.h:23:8: error: redefinition of 'struct in6_addr'
   23 | struct in6_addr {
      |        ^~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/19931aba8bee8867a53130ca4a53cfadbc4c5bfd



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>