zsmalloc: use preempt.h for in_interrupt()

A cosmetic change.

Commit c60369f0 ("staging: zsmalloc: prevent mappping in interrupt
context") added in_interrupt() check to zs_map_object() and 'hardirq.h'
include; but in_interrupt() macro is defined in 'preempt.h' not in
'hardirq.h', so include it instead.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -59,7 +59,7 @@
 #include <linux/cpumask.h>
 #include <linux/cpu.h>
 #include <linux/vmalloc.h>
-#include <linux/hardirq.h>
+#include <linux/preempt.h>
 #include <linux/spinlock.h>
 #include <linux/types.h>
 #include <linux/debugfs.h>