nut: Avoid building docs

Reduce compilation time and complexity by not building documentation
we don't use.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>

net/nut/Makefile

@@ -526,6 +526,7 @@ CONFIGURE_ARGS += \
 	--without-avahi \
 	--$(if $(CONFIG_NUT_DRIVER_SNMP),with,without)-snmp \
 	--$(if $(CONFIG_NUT_DRIVER_SERIAL),with,without)-serial \
+	--without-doc \
 	--without-neon \
 	--without-powerman \
 	--without-wrap \