[MIPS] Use -Werror on subdirectories which build cleanly.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/sni/Makefile b/arch/mips/sni/Makefile
index 471418e..3a99cd6 100644
--- a/arch/mips/sni/Makefile
+++ b/arch/mips/sni/Makefile
@@ -4,3 +4,5 @@
 
 obj-y += irq.o reset.o setup.o a20r.o rm200.o pcimt.o pcit.o time.o
 obj-$(CONFIG_CPU_BIG_ENDIAN) += sniprom.o
+
+EXTRA_CFLAGS += -Werror