spi: spi-txx9: Fix warning by including linux/gpio.h

According to the following commit direct inclusion of asm/gpio.h is deprecated. 7563bbf89d065a2c3f05059ecbcc805645edcc62 ("gpiolib/arches: Centralise bolierplate asm/gpio.h")

This patch fixes the below warnings by including linux/gpio.h.

arch/x86/include/asm/gpio.h:2:2: warning:
Include linux/gpio.h instead of asm/gpio.h

In file included from drivers/spi/spi-txx9.c:29:0:
arch/x86/include/asm/gpio.h:2:2: warning:
#warning Include linux/gpio.h instead of asm/gpio.h [-Wcpp]

---
 drivers/spi/spi-txx9.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index e9b7681..7c6d157 100644
@@ -26,7 +26,7 @@
 #include <linux/clk.h>
 #include <linux/io.h>
 #include <linux/module.h>
-#include <asm/gpio.h>
+#include <linux/gpio.h>
 
 
 #define SPI_FIFO_SIZE 4

1.7.10.4

This is already fixed.