arm: fix compilation with CONFIG_FDT

A conflict was resolved the wrong way when merging commit 320ba5f (build:
always link device_tree.o into emulators if libfdt available, 2013-02-05).
This causes a build failure for the arm-softmmu target due to multiply
defined symbol.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1362997886-9470-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>