sim: ppc: fix Wnonnull warning

When compiling with --enable-werror and CFLAGS="-O0 -g -Wall", we run into:
...
src/sim/ppc/emul_netbsd.c: In function 'do_gettimeofday':
src/sim/ppc/emul_netbsd.c:770:16: error: null argument where non-null \
  required (argument 1) [-Werror=nonnull]
   int status = gettimeofday((t_addr != 0 ? &t : NULL),
                ^~~~~~~~~~~~
...

Fix this by unconditionally passing &t as first argument.