LMIC_DEBUG_PRINTF is handy, but it needs to include a declaration

[Frank Leon Rose]

If you actually define -DLMIC_DEBUG_PRINTF MyLmicDebugPrintf you get warnings in compilation because it isn't declared in the LMiC modules.

I recommend adding a declaration immediately after #define

     #define LMIC_DEBUG_PRINTF(f, ...) LMIC_DEBUG_PRINTF_FN(f, ## __VA_ARGS__)
     extern void LMIC_DEBUG_PRINTF(const char *fmt, ...);

[Terry Moore]

Fixed by 42dfb39