acpica: work around flex 2.6.2 code generation issue

acpica: work around flex 2.6.2 code generation issue

Without this patch, linking fails with a missing implementation of
yy_scan_string. This looks like a regression in flex, because 2.6.0 generated
different code that called PrParser_scan_string
resp. DtParser_scan_string.

Working around that in acpica until this is better understood or fixed
in flex is the easiest solution for now.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>