kunit: replace kernel.h with the necessary inclusions

When kernel.h is used in the headers it adds a lot into dependency hell,
especially when there are circular dependencies are involved.

Replace kernel.h inclusion with the list of what is really being used.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Brendan Higgins <[e-mail]>
Tested-by: Brendan Higgins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/kunit/assert.h

···
 #define _KUNIT_ASSERT_H
 
 #include <linux/err.h>
-#include <linux/kernel.h>
+#include <linux/printk.h>
 
 struct kunit;
 struct string_stream;