Add explanatory comment for r327622: clang 6.0.0 and higher warn about
the ACPI_ROOT_OBJECT and ACPI_TO_POINTER macros from acpica's actypes.h
header, that they use arithmetic on a null pointer treated as a cast
from integer to pointer, which is a GNU extension. We turn off the
warning, because this is in contributed code.

Requested by: rakuco