Update getpinmap_thisboard.cpp

[cyberman54]

suppress compiler warning if using custom pin mapping

[frazar]

Would it make sense to remove the message altogether?
Compilation would fail anyway if lmic_pins is not defined, so the user would know.

[terrillmoore]

I added this message because if things are misconfigured, beginning users may get a runtime failure due to a nullptr dereference without any way to troubleshoot. #pragma message() is not portable. I agree that if the warning is being emitted when you know what you're doing, it's also annoying, especially if you're not using the routine. It would be better to change this to an LMIC_DEBUG() that warns the user before returning NULL. However, I will merge this now and file a new bug report, as there is no backward-compatibility issue.