Not defining USBCON broke since 1.8.14

[MSmeets94]

We have a PlatformIO project which has a custom board without USB. For this we don't have USBCON defined which worked fine in v1.8.13

Since PR #713 this broke because USBAPI.h now uses LineInfo which is not defined.

The definition for LineInfo is in CDC.h , surrounded by an ifdef CDC_ENABLED (which is false, since USBCON is not defined), however since CDC.cpp does always include USBAPI.h this problem arises;

I originally created this issue in platformio/platform-atmelsam#234 , but this indeed not an issue in PlatformIO but ArduinoCore