Enable FTRACE to get tracing working on arm64

Note: as of now, this kernel can be built only on ubuntu 14.04

Bug: 272339637

Signed-off-by: Bo Hu <bohu@google.com>
Change-Id: I45d0f724d132d59d53fc0ab41ce1f2193220bc3d
diff --git a/arch/arm64/configs/ranchu_defconfig b/arch/arm64/configs/ranchu_defconfig
index 0d494cd..b73e9f2 100644
--- a/arch/arm64/configs/ranchu_defconfig
+++ b/arch/arm64/configs/ranchu_defconfig
@@ -536,3 +536,5 @@
 CONFIG_VIRTIO_PCI=y
 CONFIG_GENERIC_PCI_IOMAP=y
 CONFIG_ENABLE_DEFAULT_TRACERS=y
+
+CONFIG_FTRACE=y