Fix kpatch.ko build on Ubuntu Saucy.

The error likes:
make -C /lib/modules/3.11.0-12-generic/build M=/home/ryan/kpatch/kmod/core kpatch.ko
make[3]: Entering directory `/usr/src/linux-headers-3.11.0-12-generic'
  CC [M]  /home/ryan/kpatch/kmod/core/core.o
/home/ryan/kpatch/kmod/core/core.c:42:32: fatal error: linux/preempt_mask.h: No such file or directory
 #include <linux/preempt_mask.h>

I feel sorry to introduce this problem from my laster commit 6c2d6444.
Some old kernel doesn't have header file preempt_mask.h, and the safe
way is using hardirq.h to find in_nmi().

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>