samples: big_http_download: Set min_flash to 128K.

Previously there was just "min_ram: 32", which took care about
suitable flash size implicitly. But now we have interesting boards
(e.g. lpcxpresso54114_m0) which tout RAM of 32K and Flash of 64K.
So, become more explicit about Flash requirement for the sample.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>