fpga: lattice-sysconfig-spi: Add explicit include for of.h

With linux/acpi.h (in spi/spi.h) no longer implicitly including of.h,
add an explicit include of of.h to fix the following errors:

drivers/fpga/lattice-sysconfig-spi.c:146:35: error: implicit declaration of function 'of_match_ptr' [-Werror=implicit-function-declaration]
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/fpga/lattice-sysconfig-spi.c

@@ -3,6 +3,7 @@
  * Lattice FPGA programming over slave SPI sysCONFIG interface.
  */
 
+#include <linux/of.h>
 #include <linux/spi/spi.h>
 
 #include "lattice-sysconfig.h"