arm64: Fix build error with !SMP

The __atomic_hash is only defined when SMP is enabled but the
arm64ksyms.c exports it even for the UP case.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/arm64ksyms.c

@@ -40,7 +40,9 @@ EXPORT_SYMBOL(__copy_to_user);
 EXPORT_SYMBOL(__clear_user);
 
 	/* bitops */
+#ifdef CONFIG_SMP
 EXPORT_SYMBOL(__atomic_hash);
+#endif
 
 	/* physical memory */
 EXPORT_SYMBOL(memstart_addr);