ar71xx: add missing include for checking kernel version

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: 318e19ba6755 ("ar71xx: add v4.14 support")
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(backported from 743654f30dde7659d11280e9cd34da2f1ddad0bc)