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

Commit b42faeee718c ("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 <[e-mail]>
Tested-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi.c

···
 #include <linux/highmem.h>
 #include <linux/idr.h>
 #include <linux/platform_data/x86/apple.h>
+#include <linux/ptp_clock_kernel.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/spi.h>

include/linux/spi/spi.h

···
 #include <linux/completion.h>
 #include <linux/scatterlist.h>
 #include <linux/gpio/consumer.h>
-#include <linux/ptp_clock_kernel.h>
 
 #include <uapi/linux/spi/spi.h>
 
 struct dma_chan;
 struct software_node;
+struct ptp_system_timestamp;
 struct spi_controller;
 struct spi_transfer;
 struct spi_controller_mem_ops;