Compile i8254 only once

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/default-configs/ppc64-softmmu.mak b/default-configs/ppc64-softmmu.mak
index f5b76b6..38d934e 100644
--- a/default-configs/ppc64-softmmu.mak
+++ b/default-configs/ppc64-softmmu.mak
@@ -6,3 +6,4 @@
 CONFIG_ESCC=y
 CONFIG_M48T59=y
 CONFIG_VGA_PCI=y
+CONFIG_I8254=y