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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/dec/Makefile b/arch/mips/dec/Makefile
index 9eb2f9c..c530208e 100644
--- a/arch/mips/dec/Makefile
+++ b/arch/mips/dec/Makefile
@@ -8,3 +8,5 @@
 obj-$(CONFIG_PROM_CONSOLE)	+= promcon.o
 obj-$(CONFIG_TC)		+= tc.o
 obj-$(CONFIG_CPU_HAS_WB)	+= wbflush.o
+
+EXTRA_CFLAGS += -Werror