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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/mm/Makefile b/arch/mips/mm/Makefile
index 19a0e54..43e4810 100644
--- a/arch/mips/mm/Makefile
+++ b/arch/mips/mm/Makefile
@@ -32,3 +32,5 @@
 obj-$(CONFIG_R5000_CPU_SCACHE)  += sc-r5k.o
 obj-$(CONFIG_RM7000_CPU_SCACHE)	+= sc-rm7k.o
 obj-$(CONFIG_MIPS_CPU_SCACHE)	+= sc-mips.o
+
+EXTRA_CFLAGS += -Werror