ARM: highmem: Fix cache_is_vivt() reference

The reference to cache_is_vivt() was moved into a header file,
which now causes a build failure in rare randconfig builds:

arch/arm/include/asm/highmem.h:52:43: error: implicit declaration of function 'cache_is_vivt' [-Werror,-Wimplicit-function-declaration]

Add an explicit include to make it build reliably.

Fixes: 2a15ba82 ("ARM: highmem: Switch to generic kmap atomic")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20201204165930.3877571-1-arnd@kernel.org