Update Marlin_main.cpp

Corrected "servo.h" to "Servo.h" because of the updated filename.

Marlin/Marlin_main.cpp

 #endif
 
 #if NUM_SERVOS > 0
-  #include "servo.h"
+  #include "Servo.h"
 #endif
 
 #if HAS_DIGIPOTSS