Adding preempt_mask.h to core.c

When compiling core.c, it may report error like:
"error: implicit declaration of function ‘in_nmi’"

Adding header file in_nmi defined could avoid this.

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>
Merge pull request #141 from ryanmiao/for-issue-140

Adding preempt_mask.h to core.c