trace: fix C++ compilation

trace: fix C++ compilation

trace_mem is declared as 'void *' which triggers following error:
'...invalid conversion from ‘void*’ to ‘__rte_trace_header*’
[-fpermissive]...'

Fix this by adding proper typecast to 'struct __rte_trace_header *'.

Fixes: ebaee6409702 ("trace: simplify trace point headers")
Cc: stable@dpdk.org
Signed-off-by: Pawel Wodkowski <pawelwod@gmail.com>
Signed-off-by: Sunil Kumar Kori <skori@marvell.com>
Acked-by: Nicolas Chautru <nicolas.chautru@intel.com>