spi: tools: make a symbolic link to the header file spi.h

spi: tools: make a symbolic link to the header file spi.h

The header file spi.h in include/uapi/linux/spi is needed for spidev.h,
so we also need make a symbolic link to it to eliminate the error message
as below:

In file included from spidev_test.c:24:
include/linux/spi/spidev.h:28:10: fatal error: linux/spi/spi.h: No such file or directory
   28 | #include <linux/spi/spi.h>
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.

Fixes: f7005142dace ("spi: uapi: unify SPI modes into a single spi.h")
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Link: https://lore.kernel.org/r/20210422102604.3034217-1-quanyang.wang@windriver.com
Signed-off-by: Mark Brown <broonie@kernel.org>