lib: math: polynomial: don't use 'proxy' headers

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

Link: https://lkml.kernel.org/r/20260302092831.2267785-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/math/polynomial.c

@@ -10,7 +10,8 @@
  *
  */
 
-#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/math.h>
 #include <linux/module.h>
 #include <linux/polynomial.h>