gbm: remove define _DEFAULT_SOURCE

The build systems already add this as applicable. There's no need to
have this in the source file.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gbm/main/gbm.c

@@ -25,8 +25,6 @@
  *    Benjamin Franzke <benjaminfranzke@googlemail.com>
  */
 
-#define _DEFAULT_SOURCE
-
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>