build failure on armhf and ppc64el

[arighi]

I'm getting the following build error on armhf and ppc64 with linux 5.15:

 CC [M]  /var/lib/dkms/bbswitch/0.8/build/bbswitch.o
/var/lib/dkms/bbswitch/0.8/build/bbswitch.c: In function ‘bbswitch_off’:
/var/lib/dkms/bbswitch/0.8/build/bbswitch.c:271:13: error: implicit declaration of function ‘acpi_bus_get_device’; did you mean ‘acpi_get_gpe_device’? [-Werror=implicit-function-declaration]
  271 |         r = acpi_bus_get_device(dis_handle, &ad);
      |             ^~~~~~~~~~~~~~~~~~~
      |             acpi_get_gpe_device
/var/lib/dkms/bbswitch/0.8/build/bbswitch.c:276:15: error: invalid use of undefined type ‘struct acpi_device’
  276 |         if (ad->power.state == ACPI_STATE_UNKNOWN) {
      |               ^~
/var/lib/dkms/bbswitch/0.8/build/bbswitch.c:278:15: error: invalid use of undefined type ‘struct acpi_device’
  278 |             ad->power.state = ACPI_STATE_D0;
      |               ^~

This is because in bbswitch_off() we use acpi_bus_get_device() to determine the power state, but this feature is only available on x86 and arm64. We should probably ifdef that chunk of code and use it only on x86 and arm64.

[arighi]

Created PR #214