Declare MSerial6 (etc.) for STM32 (#18149)

Marlin/src/HAL/STM32/MarlinSerial.h

@@ -52,3 +52,9 @@ extern MarlinSerial MSerial2;
 extern MarlinSerial MSerial3;
 extern MarlinSerial MSerial4;
 extern MarlinSerial MSerial5;
+extern MarlinSerial MSerial6;
+extern MarlinSerial MSerial7;
+extern MarlinSerial MSerial8;
+extern MarlinSerial MSerial9;
+extern MarlinSerial MSerial10;
+extern MarlinSerial MSerialLP1;