Fix MK3 compilation issue with Arduino 1.8.x

see https://github.com/prusa3d/Prusa-Firmware/issues/614#issuecomment-385034066

Firmware/xyzcal.cpp

@@ -32,7 +32,7 @@
 
 #define _PI 3.14159265F
 
-extern long count_position[NUM_AXIS];
+extern volatile long count_position[NUM_AXIS];
 
 uint8_t check_pinda_0();
 uint8_t check_pinda_1();