Fix custom_crash_callback declaration for Arduino IDE 1.8.6 (#1169)

code/espurna/config/prototypes.h

@@ -31,6 +31,9 @@ extern "C" {
 // -----------------------------------------------------------------------------
 void debugSend(const char * format, ...);
 void debugSend_P(PGM_P format, ...);
+extern "C" {
+     void custom_crash_callback(struct rst_info*, uint32_t, uint32_t);
+}
 
 // -----------------------------------------------------------------------------
 // Domoticz