security/wpa_supplicant: Add option to build libwpa_client

CONFIG_BUILD_WPA_CLIENT_SO allows building the libwpa_client shared
library and the wpa_ctrl.h header file. This library provides helpers
to interact with the wpa_supplicant control interface.

This is required for net/wutil.

Approved by: cy (maintainer timeout)
PR: 288933
Sponsored by: Google LLC (GSoC 2025)