Fix M49 console hang

Marlin/Marlin_main.cpp

   inline void gcode_M49() {
     g26_debug_flag ^= true;
     SERIAL_PROTOCOLPGM("G26 Debug ");
-    serialprintPGM(g26_debug_flag ? PSTR("on.") : PSTR("off."));
+    serialprintPGM(g26_debug_flag ? PSTR("on.\n") : PSTR("off.\n"));
   }
 
 #endif // G26_MESH_VALIDATION