tests: Document building of hostapd_cli for hwsim builds

This is used in the tests, too, and was already covered by the build.sh
script, but not this README file.

Signed-off-by: Vasyl Vavrychuk <vvavrychuk@gmail.com>

tests/hwsim/README

@@ -45,7 +45,7 @@ make
 cd ../hostapd
 cp ../tests/hwsim/example-hostapd.config .config
 make clean
-make hostapd hlr_auc_gw
+make hostapd hostapd_cli hlr_auc_gw
 cd ../wlantest
 make clean
 make