add extern "C" to matrix.cpp

keyboards/converter/usb_usb/matrix.cpp

@@ -83,6 +83,8 @@ KBDReportParser kbd_parser3;
 KBDReportParser kbd_parser4;
 
 
+extern "C"
+{
     uint8_t matrix_rows(void) { return MATRIX_ROWS; }
     uint8_t matrix_cols(void) { return MATRIX_COLS; }
     bool matrix_has_ghost(void) { return false; }
@@ -232,3 +234,5 @@ void led_set(uint8_t usb_led)
         kbd3.SetReport(0, 0, 2, 0, 1, &usb_led);
         kbd4.SetReport(0, 0, 2, 0, 1, &usb_led);
     }
+
+};