package/tinc: optionally include linux/if_tun.h to fix musl build

Fixes
http://autobuild.buildroot.net/results/5b1/5b1d0c8c1bd75f7e228c313cb21b1638301568ae/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>