nvmem: include linux/err.h from header

missing err.h header file can cause simillar errors like below
in some configurations.
"error: implicit declaration of function 'ERR_PTR'
[-Werror=implicit-function-declaration]"

This adds the missing include to ensure we can always include
the header.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/nvmem-consumer.h

@@ -12,6 +12,9 @@
 #ifndef _LINUX_NVMEM_CONSUMER_H
 #define _LINUX_NVMEM_CONSUMER_H
 
+#include <linux/err.h>
+#include <linux/errno.h>
+
 struct device;
 struct device_node;
 /* consumer cookie */