otautil: Fix mac build.

bootable/recovery/otautil/SysUtil.cpp:103:19: error: use of undeclared identifier 'mmap64'; did you mean 'mmap'?
  void* reserve = mmap64(nullptr, blocks * blksize, PROT_NONE, MAP_PRIVATE | MAP_ANON, -1, 0);
                  ^~~~~~

Test: mmma bootable/recovery
Change-Id: I22d7dc4d994069201e5a633cec21421e2c4182fa