samples: sockets: http_get: increase main thread stack size

After commit eeb15aa3 ("timer: hpet: enable 64 bit mode for
better usages") was applied, main thread stack usage on qemu_x86
platform increased from 984 to 1040 bytes.

Default stack size, which is 1024, is no longer enough. Change that to
1536 to make sure this sample runs correctly on qemu_x86.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>