hw/i386/acpi-build: Fix maybe-uninitialized error when ACPI hotplug off

hw/i386/acpi-build: Fix maybe-uninitialized error when ACPI hotplug off

GCC 9.3.0 thinks that 'method' can be left uninitialized. This code
is already in the "if (bsel || pcihp_bridge_en)" block statement,
but it isn't smart enough to figure it out.

Restrict the code to be used only in the "if (bsel || pcihp_bridge_en)"
block statement to fix (on Ubuntu):

  ../hw/i386/acpi-build.c: In function 'build_append_pci_bus_devices':
  ../hw/i386/acpi-build.c:496:9: error: 'method' may be used uninitialized
  in this function [-Werror=maybe-uninitialized]
    496 |         aml_append(parent_scope, method);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Fixes: df4008c9c59 ("piix4: don't reserve hw resources when hotplug is off globally")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20201108204535.2319870-4-philmd@redhat.com>
Message-Id: <20201110192316.26397-8-alex.bennee@linaro.org>