samples: sockets: big_http_download: Increase stack size by 128 bytes

Without it, the sample crashes for qemu_x86 with:

[00:00:03.080,000] <err> os: Page fault at address 0x7275632f
  (error code 0x10)
[00:00:03.080,000] <err> os: Linear address not present in page tables
[00:00:03.080,000] <err> os: PDPTE: Non-present
...

Increase by reasonable, but small amount, to keep watching stack usage
growth trends in Zephyr.

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