Fix MKS Base + Digipot compile error (#16636)

Marlin/src/HAL/HAL_LPC1768/include/digipot_mcp4451_I2C_routines.c

 #endif
 
 #include "digipot_mcp4451_I2C_routines.h"
-#include "i2c_util.h"
 
 // These two routines are exact copies of the lpc17xx_i2c.c routines.  Couldn't link to
 // to the lpc17xx_i2c.c routines so had to copy them into this file & rename them.

Marlin/src/HAL/HAL_LPC1768/include/digipot_mcp4451_I2C_routines.h

 #include <lpc17xx_i2c.h>
 #include <lpc17xx_pinsel.h>
 #include <lpc17xx_libcfg_default.h>
+#include "i2c_util.h"
 
 uint8_t digipot_mcp4451_start(uint8_t sla);
 uint8_t digipot_mcp4451_send_byte(uint8_t data);

Marlin/src/HAL/HAL_LPC1768/include/i2c_util.h

 #include <lpc17xx_pinsel.h>
 #include <lpc17xx_libcfg_default.h>
 
+#ifdef __cplusplus
+  extern "C" {
+#endif
+
 void configure_i2c(const uint8_t clock_option);
+
+#ifdef __cplusplus
+  }
+#endif

Marlin/src/feature/digipot/digipot_mcp4451.cpp

 
 void digipot_i2c_init() {
   #if MB(MKS_SBASE)
-    configure_i2c();
+    configure_i2c(16); // Setting clock_option to 16 ensure the I2C bus is initialized at 400kHz
   #else
     Wire.begin();
   #endif