sh: Add -Werror for clean directories.

Follow the MIPS and sparc64 changes for -Werror instrumentation.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/arch/sh/mm/Makefile b/arch/sh/mm/Makefile
index ee30fb4..aa44607 100644
--- a/arch/sh/mm/Makefile
+++ b/arch/sh/mm/Makefile
@@ -33,3 +33,5 @@
 obj-$(CONFIG_HUGETLB_PAGE)	+= hugetlbpage.o
 obj-$(CONFIG_32BIT)		+= pmb.o
 obj-$(CONFIG_NUMA)		+= numa.o
+
+EXTRA_CFLAGS += -Werror