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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/cobalt/Makefile b/arch/mips/cobalt/Makefile
index c292f80..a043f93 100644
--- a/arch/mips/cobalt/Makefile
+++ b/arch/mips/cobalt/Makefile
@@ -7,3 +7,5 @@
 obj-$(CONFIG_PCI)		+= pci.o
 obj-$(CONFIG_EARLY_PRINTK)	+= console.o
 obj-$(CONFIG_MTD_PHYSMAP)	+= mtd.o
+
+EXTRA_CFLAGS += -Werror