build: drop duplicate C includes

- `stdlib.h` and `string.h` is included via `curl_setup_once.h`,
  allowing to drop from `tests/server` sources and `tool_doswin.c`.

- `stdlib.h` is also included via `setup-vms.h` (earlier than above),
  allowing to drop it from `curl_addrinfo.h` on VMS.

Closes #20303

src/tool_doswin.c

@@ -30,7 +30,6 @@
 #endif
 
 #ifdef _WIN32
-#  include <stdlib.h>
 #  include <tlhelp32.h>
 #endif