installd: fix build with ToT LLVM

Fixes:

frameworks/native/cmds/installd/otapreopt_chroot.cpp:176:17: error: use of undeclared identifier 'CLONE_NEWNS'
  176 |     if (unshare(CLONE_NEWNS) != 0) {
      |                 ^

Test: presubmit
Change-Id: I66251394098c4d70ce5cb09082fa707a7f8539c5