Fix double declaration of _XOPEN_SOURCE

The adminusers.c file declared it twice; first explicitly and then
implicitly by including cdefs.h.

include/maxscale/cdefs.h

@@ -20,7 +20,7 @@
  * - Its purpose is the same as that of x86_64-linux-gnu/sys/cdefs.h, that is,
  *   it defines things that are dependent upon the compilation environment.
  * - Since this *must* be included as the very first header by all other MaxScale
- *   headers, it allows you to redfine things globally, should that be necessary,
+ *   headers, it allows you to redefine things globally, should that be necessary,
  *   for instance, when debugging something.
  * - Global constants applicable across the line can be defined here.
  */

server/core/adminusers.c

@@ -10,13 +10,11 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+#include <maxscale/cdefs.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
-#ifndef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 700
-#endif
 #include <unistd.h>
 #include <crypt.h>
 #include <maxscale/users.h>