spi: spidev_test: fix build with musl libc

spidev.h uses _IOC_SIZEBITS directly. musl libc does not provide this macro
unless linux/ioctl.h is included explicitly. Fixes build failures like:

In file included from .../host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/sys/ioctl.h:7:0,
                 from .../build/spidev_test-v3.15/spidev_test.c:20:
.../build/spidev_test-v3.15/spidev_test.c: In function ‘transfer’:
.../build/spidev_test-v3.15/spidev_test.c:75:18: error: ‘_IOC_SIZEBITS’ undeclared (first use in this function)
  ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);
                  ^

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@kernel.org>