regmap: Add another missing header for !CONFIG_REGMAP stubs

The use of WARN_ON() needs the definitions from bug.h, without it
you can get:

include/linux/regmap.h: In function 'regmap_write':
include/linux/regmap.h:525:2: error: implicit declaration of function 'WARN_ONCE' [-Werror=implicit-function-declaration]
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org

include/linux/regmap.h

@@ -16,6 +16,7 @@
 #include <linux/list.h>
 #include <linux/rbtree.h>
 #include <linux/err.h>
+#include <linux/bug.h>
 
 struct module;
 struct device;