Add pre-calculated planner.e_factor

Marlin/gcode.h

@@ -44,10 +44,6 @@
   #include "serial.h"
 #endif
 
-#if ENABLED(INCH_MODE_SUPPORT)
-  extern bool volumetric_enabled;
-#endif
-
 /**
  * GCode parser
  *
@@ -76,6 +72,8 @@ public:
 
   // Global states for GCode-level units features
 
+  static bool volumetric_enabled;
+
   #if ENABLED(INCH_MODE_SUPPORT)
     static float linear_unit_factor, volumetric_unit_factor;
   #endif