arm64: fix Kconfig include path

They're not relative. My builder failed silently and never notified me of the
error. Sigh.

Signed-off-by: Olof Johansson <olof@lixom.net>
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 430e7fa..b7b9cea 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -160,7 +160,7 @@
 
 source "kernel/Kconfig.freezer"
 
-source "Kconfig.platforms"
+source "arch/arm64/Kconfig.platforms"
 
 menu "Bus support"