🐛 Add 'static' to fix 'duplicates' (#22826)

Marlin/src/HAL/AVR/u8g_com_HAL_AVR_sw_spi.cpp

@@ -64,8 +64,8 @@
 
 #include <U8glib-HAL.h>
 
-uint8_t u8g_bitData, u8g_bitNotData, u8g_bitClock, u8g_bitNotClock;
-volatile uint8_t *u8g_outData, *u8g_outClock;
+static uint8_t u8g_bitData, u8g_bitNotData, u8g_bitClock, u8g_bitNotClock;
+static volatile uint8_t *u8g_outData, *u8g_outClock;
 
 static void u8g_com_arduino_init_shift_out(uint8_t dataPin, uint8_t clockPin) {
   u8g_outData = portOutputRegister(digitalPinToPort(dataPin));