build: Don't generate symlinks in include/ dir

As we start refactoring the code to break dependencies (see doc/refactoring.txt),
I want us to use full paths in the includes (eg, #include "base/data-struct/list.h").
This makes it more obvious when we're breaking abstraction boundaries, eg, including a file in
metadata/ from base/

lib/properties/prop_common.c

@@ -12,8 +12,8 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "lib.h"
-#include "prop_common.h"
+#include "lib/misc/lib.h"
+#include "lib/properties/prop_common.h"
 
 int prop_not_implemented_get(const void *obj, struct lvm_property_type *prop)
 {