package/wpa_supplicant: add config option to build wpa_client library

Version 2.5 of wpa_supplicant introduces the config option
CONFIG_BUILD_WPA_CLIENT_SO to enable building wpa_client as shared library.

We need to add a patch from upstream to fix a runtime error with
libwpa_client.so:
  Error relocating /usr/lib/libwpa_client.so: bin_clear_free: symbol not found

[Thomas: renumber patch.]

Cc: Martin Bark <martin@barkynet.com>
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Reviewed-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>