microblaze/mm/highmem: Add dropped #ifdef back

The conversion to generic kmap atomic broke microblaze by removing the
build fail.

Add it back.

Fixes: 7ac1b26b0a ("microblaze/mm/highmem: Switch to generic kmap atomic")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Michal Simek <monstr@monstr.eu>

arch/microblaze/mm/init.c

@@ -49,6 +49,7 @@ unsigned long lowmem_size;
 EXPORT_SYMBOL(min_low_pfn);
 EXPORT_SYMBOL(max_low_pfn);
 
+#ifdef CONFIG_HIGHMEM
 static void __init highmem_init(void)
 {
 	pr_debug("%x\n", (u32)PKMAP_BASE);