MIPS: cpu/mips/Makefile: Add a missing START line

In the commit 79b51ff8205f0354d5300570614c1d2db499679c ([MIPS] cpu/mips/
Makefile: Split [CS]OBJS onto separate lines), I wrongly deleted a START
line.  This patch puts it back.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>

cpu/mips/Makefile

diff --git a/cpu/mips/Makefile b/cpu/mips/Makefile
index 5091781..28a1cbb 100644
--- a/cpu/mips/Makefile
+++ b/cpu/mips/Makefile
@@ -25,6 +25,7 @@
 
 LIB	= $(obj)lib$(CPU).a
 
+START	= start.o
 SOBJS-y	= cache.o
 COBJS-y	= cpu.o interrupts.o