Mark unchanging arrays as const (PR#2494)

The compiler may be able to optimize if it knows an array won’t be
changing.

Marlin/Marlin.h

@@ -241,7 +241,6 @@ inline void refresh_cmd_timeout() { previous_cmd_ms = millis(); }
   #define CRITICAL_SECTION_END    SREG = _sreg;
 #endif
 
-extern float homing_feedrate[];
 extern bool axis_relative_modes[];
 extern int feedrate_multiplier;
 extern bool volumetric_enabled;