Move glib and gtk includes out of extern C for tests.

Needed to compile with glib 2.67 and later.

libgnucash/engine/test/test-numeric.cpp

@@ -21,12 +21,12 @@
  *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  *  02110-1301, USA.
  */
+#include <glib.h>
 
 extern "C"
 {
 #include <config.h>
 #include <ctype.h>
-#include <glib.h>
 #include "cashobjects.h"
 #include "test-stuff.h"
 #include "test-engine-stuff.h"