spi: Add a prototype for exported spi_set_cs_timing()

Compiler is not happy about spi_set_cs_timing() prototype.

drivers/spi/spi.c:3016:6: warning: no previous prototype for ‘spi_set_cs_timing’ [-Wmissing-prototypes]
 void spi_set_cs_timing(struct spi_device *spi, u8 setup, u8 hold,
      ^~~~~~~~~~~~~~~~~

Let's add it to the header.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>