freetdm: define _GNU_SOURCE to get gethostbyname_r

libs/freetdm/src/ftmod/ftmod_sangoma_boost/sangoma_boost_client.c

@@ -31,6 +31,8 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define _GNU_SOURCE
+
 #if HAVE_NETDB_H
 #include <netdb.h>
 #endif