bootstd: fix build error when CONFIG_MMC is disabled

This code assumes that CONFIG_MMC and it causes a build error when
the config is disabled.

aarch64-linux-gnu-ld.bfd: test/boot/bootstd_common.o: in function `bootstd_test_check_mmc_hunter':
test/boot/bootstd_common.c:83:(.text.bootstd_test_check_mmc_hunter+0x70):
    undefined reference to `_u_boot_list_2_bootdev_hunter_2_mmc_bootdev_hunter'

Fixes: 66e3dce7 ("bootstd: Allow hunting for a bootdev by label")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>