workflow_active was never defined or used

`supervisor_workflow_active` needs to be stubbed for when there's no USB

supervisor/shared/workflow.h

@@ -27,4 +27,3 @@
 #pragma once
 
 extern bool supervisor_workflow_connecting(void);
-extern bool supervisor_workflow_active(void);

supervisor/stub/serial.c

@@ -53,3 +53,7 @@ void serial_write(const char *text) {
 
 void supervisor_workflow_reset(void) {
 }
+
+bool supervisor_workflow_active(void) {
+    return false;
+}

supervisor/workflow.h

@@ -29,4 +29,4 @@
 void supervisor_workflow_reset(void);
 
 // True when the user could be actively iterating on their code.
-bool workflow_active(void);
+bool supervisor_workflow_active(void);