ixp/interrupts: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/ixp/Makefile b/cpu/ixp/Makefile
index 7e98d87..a673cb1 100644
--- a/cpu/ixp/Makefile
+++ b/cpu/ixp/Makefile
@@ -27,9 +27,10 @@
 
 START	= start.o
 COBJS-y	+= cpu.o
-COBJS-y	+= interrupts.o
 ifndef CONFIG_USE_IRQ
 COBJS-y	+= timer.o
+else
+COBJS-y	+= interrupts.o
 endif
 
 SRCS	:= $(START:.o=.S) $(SOBJS-y:.o=.S) $(COBJS-y:.o=.c)