samples: http_get: increase mbedTLS heap size

40k heap size is not enough and results in handshake errors:

  <err> net_sock_tls: TLS handshake error: -0x2700

Increasing it to 55k already works (when used with native_sim_64 and
offloaded sockets), but use 60k to get a bit more room for future.

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