ip_frag: fix header for C++
ip_frag: fix header for C++

Add missing extern 'C' decls in rte_ip_frag.h.

Fixes: 601e279df074 ("move fragmentation/reassembly headers into a library")

Signed-off-by: Marc Sune <8ac2233f04d2956a682020530ebe37f0d9d46630@bisdn.de>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk