rust/minijail: Fix cast to c_char. am: 11395a6982 am: 036c252f23 am: af243930d7

Original change: https://android-review.googlesource.com/c/platform/external/minijail/+/1682968

Change-Id: I0f06081d9db9e4bf0493f66ce76731c4807dd6c1