spidev_test: fix musl build

spidev.h needs _IOC_SIZEBITS. Add linux/ioctl.h for that to fix build under
musl.

The patch suggested upstream (https://patchwork.kernel.org/patch/9276909/)
does not apply to the spidev_test.c version we currently use. A backported
patch would break once we bump spidev_test.c. A bump+patch solution would
break if/when upstream accepts this patch. That's why this commit uses the
unusual sed patching to keep compatibility with current and future
spidev_test.c versions.

Fixes:
http://autobuild.buildroot.net/results/128/1284a36c70ff8a1412e2128266c6654b2aa5b2da/
http://autobuild.buildroot.net/results/0ad/0adea6121b238adf0e64d455a7573b1a31c38264/
http://autobuild.buildroot.net/results/21b/21b92651ce456f3819eef315cecd92ff70b9e98f/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>