ar71xx: add missing include for checking kernel version

Fixes these build errors:

arch/mips/ath79/mach-rb2011.c:20:5: error: "LINUX_VERSION_CODE" is not defined, evaluates to 0 [-Werror=undef]
 #if LINUX_VERSION_CODE < KERNEL_VERSION(4,14,0)
     ^~~~~~~~~~~~~~~~~~
arch/mips/ath79/mach-rb2011.c:20:26: error: "KERNEL_VERSION" is not defined, evaluates to 0 [-Werror=undef]
 #if LINUX_VERSION_CODE < KERNEL_VERSION(4,14,0)
                          ^~~~~~~~~~~~~~
arch/mips/ath79/mach-rb2011.c:20:40: error: missing binary operator before token "("
 #if LINUX_VERSION_CODE < KERNEL_VERSION(4,14,0)
                                        ^

Fixes: 318e19ba67 ("ar71xx: add v4.14 support")

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/ar71xx/files/arch/mips/ath79/mach-c60.c

@@ -12,6 +12,7 @@
  *  by the Free Software Foundation.
  */
 
+#include <linux/version.h>
 #include <linux/pci.h>
 #include <linux/phy.h>
 #include <linux/mtd/mtd.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-domywifi-dw33d.c

@@ -18,6 +18,7 @@
  *
  */
 
+#include <linux/version.h>
 #include <linux/platform_device.h>
 #include <linux/ar8216_platform.h>
 #include <linux/mtd/mtd.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-nbg6716.c

@@ -20,6 +20,7 @@
  *
  */
 
+#include <linux/version.h>
 #include <linux/platform_device.h>
 #include <linux/ar8216_platform.h>
 #include <linux/gpio.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-rb2011.c

@@ -11,6 +11,7 @@
 
 #define pr_fmt(fmt) "rb2011: " fmt
 
+#include <linux/version.h>
 #include <linux/phy.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-rb91x.c

@@ -10,6 +10,7 @@
 
 #define pr_fmt(fmt) "rb91x: " fmt
 
+#include <linux/version.h>
 #include <linux/phy.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-rb922.c

@@ -8,6 +8,7 @@
  *  by the Free Software Foundation.
  */
 
+#include <linux/version.h>
 #include <linux/phy.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-rb95x.c

@@ -12,6 +12,7 @@
 
 #define pr_fmt(fmt) "rb95x: " fmt
 
+#include <linux/version.h>
 #include <linux/phy.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-rbsxtlite.c

@@ -12,6 +12,7 @@
 
 #define pr_fmt(fmt) "sxtlite: " fmt
 
+#include <linux/version.h>
 #include <linux/phy.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

target/linux/ar71xx/files/arch/mips/ath79/mach-wndr4300.c

@@ -9,6 +9,7 @@
  *  by the Free Software Foundation.
  */
 
+#include <linux/version.h>
 #include <linux/pci.h>
 #include <linux/phy.h>
 #include <linux/gpio.h>

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_ethtool.c

@@ -12,6 +12,7 @@
  */
 
 #include "ag71xx.h"
+#include <linux/version.h>
 
 static int ag71xx_ethtool_get_settings(struct net_device *dev,
 				       struct ethtool_cmd *cmd)