Fix digipot compilation (#19731)

Marlin/src/feature/digipot/digipot.h

@@ -30,4 +30,4 @@ public:
   static void set_current(const uint8_t channel, const float current);
 };
 
-DigipotI2C digipot_i2c;
+extern DigipotI2C digipot_i2c;

Marlin/src/feature/digipot/digipot_mcp4018.cpp

@@ -99,4 +99,6 @@ void DigipotI2C::init() {
     set_current(i, pgm_read_float(&digipot_motor_current[i]));
 }
 
+DigipotI2C digipot_i2c;
+
 #endif // DIGIPOT_MCP4018

Marlin/src/feature/digipot/digipot_mcp4451.cpp

@@ -95,4 +95,6 @@ void DigipotI2C::init() {
     set_current(i, pgm_read_float(&digipot_motor_current[i]));
 }
 
+DigipotI2C digipot_i2c;
+
 #endif // DIGIPOT_MCP4451