httpc: prefer curl headers in submodule by default

FreeBSD instances in Tarantool CI have installed libcurl package (as a
dependency of Zabbix monitoring agent). Curl 8.4.0 introduces a new
function `curl_multi_get_handles` that is used in the following commit
in `src/curl.c`, but libcurl system package has no such symbol in
headers. On building on FreeBSD in Tarantool CI C compiler produces a
warning about implicit declaration of function, because it looks at
system headers by default and due to enabled CMake option
`-DENABLE_WERROR=ON` building has failed:

```
[ 63%] Building C object src/CMakeFiles/server.dir/title.c.o
/.cache/act/55d136250dd94303/hostexecutor/src/curl.c:266:17: error: implicit declaration of function 'curl_multi_get_handles' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
                CURL **list = curl_multi_get_handles(env->multi);
                              ^
/.cache/act/55d136250dd94303/hostexecutor/src/curl.c:266:17: note: did you mean 'curl_multi_add_handle'?
/usr/local/include/curl/multi.h:140:23: note: 'curl_multi_add_handle' declared here
CURL_EXTERN CURLMcode curl_multi_add_handle(CURLM *multi_handle,
                      ^
/.cache/act/55d136250dd94303/hostexecutor/src/curl.c:266:10: error: incompatible integer to pointer conversion initializing 'CURL **' (aka 'void **') with an expression of type 'int' [-Werror,-Wint-conversion]
                CURL **list = curl_multi_get_handles(env->multi);
                       ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
```

The patch fixes that by reordering headers passed to compiler, see [1].

1. https://cmake.org/cmake/help/latest/command/include_directories.html

Needed for #9283

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build

(cherry picked from commit 0a3500d3)