x86/mm: Include asm/numa.h for set_highmem_pages_init()

The set_highmem_pages_init() function is declared in asm/numa.h, which
must be included in the file that defines it to avoid a W=1 warning:

arch/x86/mm/highmem_32.c:7:13: error: no previous prototype for 'set_highmem_pages_init' [-Werror=missing-prototypes]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-9-arnd%40kernel.org

arch/x86/mm/highmem_32.c

@@ -3,6 +3,7 @@
 #include <linux/export.h>
 #include <linux/swap.h> /* for totalram_pages */
 #include <linux/memblock.h>
+#include <asm/numa.h>
 
 void __init set_highmem_pages_init(void)
 {