include/linux/highmem.h needs hardirq.h

Commit 3e4d3af5 ("mm: stack based kmap_atomic()") introduced the
kmap_atomic_idx_push() function which warns on in_irq() with
CONFIG_DEBUG_HIGHMEM enabled.  This patch includes linux/hardirq.h for
the in_irq definition.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>