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)