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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

tools/spi/Makefile

···
 #
 # We need the following to be outside of kernel tree
 #
-$(OUTPUT)include/linux/spi/spidev.h: ../../include/uapi/linux/spi/spidev.h
+$(OUTPUT)include/linux/spi: ../../include/uapi/linux/spi
 	mkdir -p $(OUTPUT)include/linux/spi 2>&1 || true
 	ln -sf $(CURDIR)/../../include/uapi/linux/spi/spidev.h $@
+	ln -sf $(CURDIR)/../../include/uapi/linux/spi/spi.h $@
 
-prepare: $(OUTPUT)include/linux/spi/spidev.h
+prepare: $(OUTPUT)include/linux/spi
 
 #
 # spidev_test