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: NCatalin Marinas <catalin.marinas@arm.com>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/highmem.h

@@ -5,6 +5,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/uaccess.h>
+#include <linux/hardirq.h>
 
 #include <asm/cacheflush.h>