ACPI / osi: Make local function acpi_osi_dmi_linux() static

The function acpi_osi_dmi_linux() is local and does not need to be in
global scope, so make it static.

Cleans up sparse warning:
"symbol 'acpi_osi_dmi_linux' was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>