tcg: add the CONFIG_TCG into Makefiles

Add the CONFIG_TCG for frontend and backend's files in the related
Makefiles.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/accel/Makefile.objs b/accel/Makefile.objs
index cd5702f..10666ed 100644
--- a/accel/Makefile.objs
+++ b/accel/Makefile.objs
@@ -1,4 +1,4 @@
 obj-$(CONFIG_SOFTMMU) += accel.o
 obj-y += kvm/
-obj-y += tcg/
+obj-$(CONFIG_TCG) += tcg/
 obj-y += stubs/