add '#define USE_SERIAL' to keyboards/mint60/config.h (#5758)

If there is no '#define USE_SERIAL', qmk configurator will give an error, so add '#define USE_SERIAL' to `mint60/config.h`。

keyboards/mint60/config.h

@@ -31,6 +31,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define TAPPING_FORCE_HOLD
 #define TAPPING_TERM 100
 
+#define USE_SERIAL
+
 /* key matrix size */
 #define MATRIX_ROWS 10
 #define MATRIX_COLS 8

keyboards/mint60/keymaps/default/config.h

@@ -17,7 +17,7 @@
 #pragma once
 
 /* Use I2C or Serial, not both */
-#define USE_SERIAL
+// #define USE_SERIAL
 // #define USE_I2C
 
 // #define MASTER_RIGHT

keyboards/mint60/keymaps/eucalyn/config.h

@@ -17,7 +17,7 @@
 #pragma once
 
 /* Use I2C or Serial, not both */
-#define USE_SERIAL
+// #define USE_SERIAL
 // #define USE_I2C
 
 // #define MASTER_RIGHT