mbuf: fix C++ build on void pointer cast

mbuf: fix C++ build on void pointer cast

Including rte_mbuf.h in C++ triggers the following warning as C++ does not
allow implicit casting of a void *.

In file included from test.cpp:1:0:
rte_mbuf.h: In function ‘rte_mbuf_ext_shared_info*
rte_pktmbuf_ext_shinfo_init_helper(void*, uint16_t*,
rte_mbuf_extbuf_free_callback_t, void*)’:
rte_mbuf.h:1349:9: error: invalid conversion
from ‘void*’ to ‘rte_mbuf_ext_shared_info*’ [-fpermissive]
  shinfo = RTE_PTR_ALIGN_FLOOR(RTE_PTR_SUB(buf_end,
         ^

Fixes: a53aa2b9f3be ("mbuf: support attaching external buffer")

Signed-off-by: David Marchand <david.marchand@6wind.com>