libctf: fix unnecessary glibcism in testsuite

libctf-regression/open-error-free.c #defines _GNU_SOURCE at its top.
This is already done by config.h if necessary: drop it.

libctf/ChangeLog:

	* testsuite/libctf-regression/open-error-free.c: Drop
          unnecessary system-dependent #define.

libctf/testsuite/libctf-regression/open-error-free.c

@@ -1,6 +1,5 @@
 /* Make sure that, on error, an opened dict is properly freed.  */
 
-#define _GNU_SOURCE 1
 #include "config.h"
 #include <dlfcn.h>
 #include <stdio.h>