BUILD: httpclient: fix build without SSL

BUILD: httpclient: fix build without SSL

src/http_client.c: In function ‘httpclient_cfg_postparser’:
src/http_client.c:1065:8: error: unused variable ‘errmsg’ [-Werror=unused-variable]
 1065 |  char *errmsg = NULL;
      |        ^~~~~~
src/http_client.c:1064:6: error: unused variable ‘err_code’ [-Werror=unused-variable]
 1064 |  int err_code = 0;
      |      ^~~~~~~~

Fix the build of the httpclient without SSL, the problem was introduced
with previous patch 71e3158 ("BUG/MINOR: httpclient: send the SNI using
the host header")

Must be backported in 2.5 as well.