acpi: ignore new gcc 9 warning

gcc 9 started to give us the following warnings:

external/x64/acpica/source/components/tables/tbfind.c:162:5: error: ‘strncpy’ specified bound 6 equals destination size [-Werror=stringop-truncation]
  162 |     ACPI_STRNCPY (Header.OemId, OemId, ACPI_OEM_ID_SIZE);
      |     ^~~~~~~~~~~~

Let's just turn of this warning for the ACPI files. We're not going
to fix problems in that library (and I'm not even sure it is a real
problem, I don't know if "OemId" is really guaranteed to be null
terminated).

Signed-off-by: Nadav Har'El <[email protected]>
Message-Id: <[email protected]>

Makefile

@@ -473,7 +473,7 @@ acpi-defines = -DACPI_MACHINE_WIDTH=64 -DACPI_USE_LOCAL_CACHE
 acpi-source := $(shell find external/$(arch)/acpica/source/components -type f -name '*.c')
 acpi = $(patsubst %.c, %.o, $(acpi-source))
 
-$(acpi:%=$(out)/%): CFLAGS += -fno-strict-aliasing
+$(acpi:%=$(out)/%): CFLAGS += -fno-strict-aliasing -Wno-stringop-truncation
 
 endif # x64