kcov: add more missing includes

It is fragile that some definitions acquired via transitive
dependencies, as shown in below:

atomic_*        (<linux/atomic.h>)
ENOMEM/EN*      (<linux/errno.h>)
EXPORT_SYMBOL   (<linux/export.h>)
device_initcall (<linux/init.h>)
preempt_*       (<linux/preempt.h>)

Include them to prevent possible issues.

Link: http://lkml.kernel.org/r/1481163221-40170-1-git-send-email-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kcov.c

@@ -1,11 +1,16 @@
 #define pr_fmt(fmt) "kcov: " fmt
 
 #define DISABLE_BRANCH_PROFILING
+#include <linux/atomic.h>
 #include <linux/compiler.h>
+#include <linux/errno.h>
+#include <linux/export.h>
 #include <linux/types.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/init.h>
 #include <linux/mm.h>
+#include <linux/preempt.h>
 #include <linux/printk.h>
 #include <linux/sched.h>
 #include <linux/slab.h>