package/socat: fix build issue

Fix two build issue when socat is build with musl:
  - Set NETDB_INTERNAL locally since it's not provided by musl
  - remove if_tun.h kernel header

Fixes:
http://autobuild.buildroot.net/results/029/02977d3844ef62b1e9cbfbab244593632757c5e2



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>