app: Add MQTT to PPP config

MQTT has been the only module disabled for PPP configuration.
There used to be at least HTTP and FTP.
Add MQTT into it so we have almost everything in one configuration.

Signed-off-by: Tommi Rantanen <tommi.rantanen@nordicsemi.no>

Author: trantanen

app/overlay-ppp.conf

@@ -6,9 +6,6 @@
 
 CONFIG_SM_PPP=y
 
-# When using PPP, disable commands of IP-based protocols to save flash space.
-CONFIG_SM_MQTTC=n
-
 # NCS modem libraries
 CONFIG_AT_CMD_CUSTOM=y