arch/powerpc/lib/bat_rw.c: Fix GCC 4.6 build warning

Fix:
bat_rw.c: In function 'write_bat':
bat_rw.c:38:6: warning: variable 'batn' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Andy Fleming <afleming@gmail.com>

arch/powerpc/lib/bat_rw.c

@@ -26,6 +26,7 @@
 #include <asm/processor.h>
 #include <asm/mmu.h>
 #include <asm/io.h>
+#include <linux/compiler.h>
 
 #ifdef CONFIG_ADDR_MAP
 #include <addr_map.h>
@@ -35,7 +36,7 @@ DECLARE_GLOBAL_DATA_PTR;
 
 int write_bat (ppc_bat_t bat, unsigned long upper, unsigned long lower)
 {
-	int batn = -1;
+	__maybe_unused int batn = -1;
 
 	sync();