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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/jmr3927/common/Makefile b/arch/mips/jmr3927/common/Makefile
index 01e7db1..8fd4fcc 100644
--- a/arch/mips/jmr3927/common/Makefile
+++ b/arch/mips/jmr3927/common/Makefile
@@ -3,3 +3,5 @@
 #
 
 obj-y	 += prom.o puts.o
+
+EXTRA_CFLAGS += -Werror