cfgt: fix implicit declaration of 'strcasestr'

> cfgt_int.c: In function 'cfgt_msgout':
> cfgt_int.c:863:21: warning: implicit declaration of function 'strcasestr'; did you mean 'strcasecmp'? [-Wimplicit-function-declaration]
>  863 |  if(buf->len > 7 && strcasestr(buf->s, "OPTIONS")) {
>      |                     ^~~~~~~~~~
>      |                     strcasecmp

src/modules/cfgt/cfgt_int.c

@@ -19,6 +19,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  *
  */
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <sys/stat.h>
 #include <dirent.h>