variants: XIAO NRF52: Enable user button

The Xiao nRF52840 combined with the Wio-SX1262 is often used for
cheap and compact DIY companion nodes. The Wio actually has an onboard
pushbutton that can be used as user button. Enable support for the
button.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

variants/xiao_nrf52/variant.h

@@ -38,7 +38,7 @@ extern "C"
 #define LED_STATE_ON            (1)     // State when LED is litted
 
 // Buttons
-#define PIN_BUTTON1             (PINS_COUNT)
+#define PIN_BUTTON1             (0)
 
 // Digital PINs
 static const uint8_t D0  = 0 ;