Fix missing string for M48 build (#16708)

Marlin/src/gcode/calibrate/M48.cpp

@@ -53,6 +53,9 @@
  *
  * This function requires the machine to be homed before invocation.
  */
+
+extern const char SP_Y_STR[];
+
 void GcodeSuite::M48() {
 
   if (axis_unhomed_error()) return;