px4io: increase main stack size by 100 bytes

when loading a mixer we were sometimes getting stack corruption in
px4io inside the sscanf() code that parses the mixers. Adding 100 more
bytes fixes this

Author: tridge

nuttx-configs/px4io-v2/nsh/defconfig

@@ -540,7 +540,7 @@ CONFIG_BOOT_COPYTORAM=n
 CONFIG_CUSTOM_STACK=n
 CONFIG_STACK_POINTER=
 CONFIG_IDLETHREAD_STACKSIZE=1024
-CONFIG_USERMAIN_STACKSIZE=1200
+CONFIG_USERMAIN_STACKSIZE=1300
 CONFIG_PTHREAD_STACK_MIN=512
 CONFIG_PTHREAD_STACK_DEFAULT=1024
 CONFIG_HEAP_BASE=