[Keyboard] Fix font file for Ein 60 Keyboard (#14553)

keyboards/ein_60/glcdfont.c

@@ -5,7 +5,7 @@
 
 #include "progmem.h"
 
-static const char PROGMEM font[] = {
+static const unsigned char PROGMEM font[] = {
     0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
     0x3E, 0x5B, 0x4F, 0x5B, 0x3E, 0x00,
     0x3E, 0x6B, 0x4F, 0x6B, 0x3E, 0x00,