tools/nolibc: fix build warning in sys_mmap() when my_syscall6 is not defined

We return -ENOSYS when there's no syscall6() operation, but we must cast
it to void* to avoid a warning.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>