WelcomeMessagePrinter.h: fixed a warning.

Appears when using the `[-Wextra]` flag.

lib/DnWiFiDoorLock/DnWiFiDoorLock/App.h:475:13: warning: missing initializer for member 'DnApp::Arduino::WelcomeMessagePrinter::Build::compiler' [-Wmissing-field-initializers]
  475 |             };
      |             ^

lib/DnAppArduino/src/DnApp/Arduino/WelcomeMessagePrinter.h

@@ -43,7 +43,7 @@ namespace DnApp::Arduino {
 
                 int versionPatchLevel = __GNUC_PATCHLEVEL__;
 #endif
-            } compiler;
+            } compiler = {};
         };
 
         explicit