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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/vr41xx/common/Makefile

@@ -3,3 +3,5 @@
 #
 
 obj-y	+= bcu.o cmu.o giu.o icu.o init.o irq.o pmu.o rtc.o siu.o type.o
+
+EXTRA_CFLAGS += -Werror