Move prototype to header file.

Firmware/Marlin.h

@@ -284,6 +284,7 @@ void setPwmFrequency(uint8_t pin, int val);
   #define CRITICAL_SECTION_END    SREG = _sreg;
 #endif //CRITICAL_SECTION_START
 
+extern bool fans_check_enabled;
 extern float homing_feedrate[];
 extern bool axis_relative_modes[];
 extern int feedmultiply;

Firmware/temperature.cpp

@@ -483,8 +483,6 @@ void countFanSpeed()
 	fan_edge_counter[1] = 0;
 }
 
-extern bool fans_check_enabled;
-
 void checkFanSpeed()
 {
 	uint8_t max_print_fan_errors = 0;

Firmware/ultralcd.cpp

@@ -41,8 +41,6 @@
 #include "static_assert.h"
 #include "io_atmega2560.h"
 
-extern bool fans_check_enabled;
-
 
 int scrollstuff = 0;
 char longFilenameOLD[LONG_FILENAME_LENGTH];