Increased PPP stack size based on testing

Increased stack size from 768 to 816.

Author: Mika Leppänen

features/netsocket/ppp/mbed_lib.json

@@ -25,7 +25,7 @@
         },
         "thread-stacksize": {
             "help": "Thread stack size for PPP",
-            "value": 768
+            "value": 816
         },
         "mbed-event-queue": {
             "help": "Use mbed event queue instead of PPP thread",