Prevent gcc from searching config.h in the current directory

This will avoid a ninja-build from picking up a config.h generated by the autotools build
(in the root build directory). Picking up the wrong config.h may lead to all kinds of
subtle issues if the autotools run was done with different options than the cmake run.

libgnucash/backend/xml/test/test-load-backend.cpp

@@ -26,7 +26,7 @@
  */
 extern "C"
 {
-#include "config.h"
+#include <config.h>
 #include "qof.h"
 #include "cashobjects.h"
 #include "test-stuff.h"