ACPICA: add type casts for string functions

Detected by gcc 10.2.0.

ACPICA commit 608559800e1ad48b819744aeb1866d94335e2655

Link: https://github.com/acpica/acpica/commit/60855980


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Erik Kaneda <erik.kaneda@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>