spi: don't include ptp_clock_kernel.h in spi.h

Commit b42faeee

 ("spi: Add a PTP system timestamp
to the transfer structure") added an include of ptp_clock_kernel.h
to spi.h for struct ptp_system_timestamp but a forward declaration
is enough. Let's use that to limit the number of objects we have
to rebuild every time we touch networking headers.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210904013140.2377609-1-kuba@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>