drm/i915/gt: include tsc.h where used

We are currently using tsc_khz as a fallback so add the right include.
For other architectures we may need to add a different fallback, but
this is not being used by dgfx so we may as well just paper it over.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211007233212.3896460-2-lucas.demarchi@intel.com