net: lib: http-client: Use memcpy() to avoid gcc warning

GCC 12.2.0 can give this warning (version 11.4.0 did not), when
CONFIG_SPEED_OPTIMIZATIONS=y

subsys/net/lib/http/http_client.c: In function 'http_send_data.constprop':
subsys/net/lib/http/http_client.c:114:33: warning: 'strncpy' specified
bound depends on the length of the source argument [-Wstringop-truncation]
  114 |                                 strncpy(send_buf + end_of_send,
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  115 |                                         data + end_of_data,
      |                                         ~~~~~~~~~~~~~~~~~~~
  116 |                                         remaining);
      |                                         ~~~~~~~~~~
subsys/net/lib/http/http_client.c:87:41: note: length computed here
   87 |                         remaining_len = strlen(data + end_of_data);
      |                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~

The code properly checks that we do not overwrite the destination
buffer even if we use the source buffer length to determine how much
to copy. One possible fix is to use memcpy() or strcpy(), I opted
to use memcpy() because it has the length option which feels more
natural.

Fixes #79326

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>