Fix 'UNUSED redefined' compiler warning (#13387)

Marlin/src/core/macros.h

 #define NANOSECONDS_PER_CYCLE (1000000000.0 / F_CPU)
 
 // Remove compiler warning on an unused variable
-#define UNUSED(x) ((void)(x))
+#define UNUSED(X) (void)X
 
 // Macros to make a string from a macro
 #define STRINGIFY_(M) #M