Fix nodemcuv2 PRINTF_TO for espressif8266 v3.0.0

Add -D _GNU_SOURCE to [env:nodemcuv2] in platformio.ini.
Espressif8266 platform v3.0.0 now also requires this,
otherwise LMIC_PRINTF_TO will fail to compile.

platformio.ini

@@ -741,6 +741,7 @@ build_flags =
     ${mcci_lmic.build_flags} 
     -D BSFILE=\"boards/bsf_nodemcuv2.h\"
     -D MONITOR_SPEED=${env:nodemcuv2.monitor_speed}
+    -D _GNU_SOURCE
     -D LMIC_PRINTF_TO=Serial    
     -D USE_SERIAL
     -D USE_LED
@@ -779,8 +780,8 @@ build_flags =
     ${mcci_lmic.build_flags} 
     -D BSFILE=\"boards/bsf_pico.h\"
     -D MONITOR_SPEED=${common.monitor_speed}
-    -D _GNU_SOURCE    
-    -D LMIC_PRINTF_TO=SerialUSB    
+    -D _GNU_SOURCE
+    -D LMIC_PRINTF_TO=SerialUSB
     -D USE_SERIAL
     -D USE_LED
     ; -D USE_DISPLAY             ; Requires external I2C OLED display