kernel.h: Move ARRAY_SIZE() to a separate header

Touching files so used for the kernel,
forces 'make' to recompile most of the kernel.

Having those definitions in more granular files
helps avoid recompiling so much of the kernel.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230817143352.132583-2-lucas.segarra.fernandez@intel.com
[andy: reduced to cover only string.h for now]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

include/linux/string.h

@@ -2,6 +2,7 @@
 #ifndef _LINUX_STRING_H_
 #define _LINUX_STRING_H_
 
+#include <linux/array_size.h>
 #include <linux/compiler.h>	/* for inline */
 #include <linux/types.h>	/* for size_t */
 #include <linux/stddef.h>	/* for NULL */