Initialize default units (#11295)

When `INCH_MODE_SUPPORT` is enabled no default unit is set. This fixes it.

Marlin/src/gcode/parser.h

   #endif
 
   #if ENABLED(DEBUG_GCODE_PARSER)
-    void debug();
+    static void debug();
   #endif
 
   // Reset is done before parsing
 
   #if ENABLED(INCH_MODE_SUPPORT)
 
+    // Init linear units by constructor
+    GCodeParser() { set_input_linear_units(LINEARUNIT_MM); }
+
     inline static void set_input_linear_units(const LinearUnit units) {
       switch (units) {
         case LINEARUNIT_INCH: