samples: big_http_download: make num_iterations unsigned

It is documented as unsigned in
samples/net/sockets/big_http_download/Kconfig

Signed overflow is undefined behavior, unsigned is not. This fixes the
following warning:

   -Wstrict-overflow=1

samples/net/sockets/big_http_download/src/big_http_download.c:346:2:
In function 'main':
error: iteration 2147483648 invokes undefined behavior
                           [-Werror=aggressive-loop-optimizations]
  346 |  } while (--num_iterations != 0);

Signed-off-by: Marc Herbert <marc.herbert@intel.com>