src/osdp_common.c: waring: _GNU_SOURCE redefined

With gcc 8.4.0 the following warning was thrown:

src/osdp_common.c:7: warning: "_GNU_SOURCE" redefined
 #define _GNU_SOURCE  /* See feature_test_macros(7) */

As the application defines using _GNU_SOURCE or not it's safe to
conditionally set this macro

src/osdp_common.c

@@ -3,8 +3,9 @@
  *
  * SPDX-License-Identifier: Apache-2.0
  */
-
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE /* See feature_test_macros(7) */
+#endif
 
 #include <stdarg.h>
 #include <stdlib.h>