Avoid "implicit declaration of function ‘strtold_l’" in wcstold.c

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/stdlib/wcstold.c

@@ -28,6 +28,7 @@ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 */
 
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <string.h>
 #include <wchar.h>