env: ENV_IS_IN_EXT4 should enable SYS_MMC_ENV_DEV

When enabling CONFIG_ENV_IS_IN_EXT4 the environment might be stored on
an mmc device similar to the CONFIG_ENV_IS_IN_FAT case.

Fixes: 7d080773 ("Convert CONFIG_SYS_MMC_ENV_DEV et al to Kconfig")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>