Fix -Wall -Werror for wpa_supplicant build

wpa_supplicant wants -Wall, -Werror, and -Wextra but needs
-Wno-unused-function and -Wno-unused-parameter to compile cleanly in
the fuchsia toolchain. This removes the default warning set and adds
the flags explicitly.

This is a bit hacky - if we make the //build configurations more
flexible in the future we should revisit.

Change-Id: Ic4ccfb11cc7ad80d912f91102c64c637c0b20f46