The new support for nvmem devices from the rtc layer caused a build
error in some configurations:
include/linux/nvmem-provider.h: In function 'nvmem_register':
include/linux/nvmem-provider.h:51:9: error: implicit declaration of function 'ERR_PTR' [-Werror=implicit-function-declaration]
This adds the missing include to ensure we can always include
the header.