lib/tests: string_helpers: don't use "proxy" headers

Update header inclusions to follow IWYU (Include What You Use) principle.

Link: https://lore.kernel.org/[email protected]
Signed-off-by: Andy Shevchenko <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

lib/tests/string_helpers_kunit.c

@@ -5,11 +5,16 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <kunit/test.h>
+
 #include <linux/array_size.h>
-#include <linux/kernel.h>
+#include <linux/bug.h>
+#include <linux/limits.h>
+#include <linux/module.h>
 #include <linux/random.h>
-#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/sprintf.h>
 #include <linux/string_helpers.h>
+#include <linux/types.h>
 
 static void test_string_check_buf(struct kunit *test,
 				  const char *name, unsigned int flags,