examples/performance-thread: fix return type of threads

examples/performance-thread: fix return type of threads

The function pthread_create() expects void *(*func) (void *)
for function pointer, however, lthread_func_t was defined as
void (*func) (void *), so now gcc 8.1 warns that the cast is
incorrect, causing a compilation failure. This patch changes
the declaration of lthread_func_t from returning a void to
returning a void*, and then changes the sample app in the
relevant places that are affected by the typedef change.

Fixes: 116819b9ed0d ("examples/performance-thread: add lthread subsystem")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>