env: mmc/fat/ext4: undefined reference to `mmc_initialize'

For CONFIG_ENV_FAT_INTERFACE != 'mmc' a link error
env/fat.c:93: undefined reference to `mmc_initialize'
occurs if CONFIG_MMC_SUPPORT is not enabled.

Fixes: 26862b4a ("env: mmc/fat/ext4: make sure that the MMC sub-system
is initialized before using it")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>