misc: Work with src != builddir

libosmo-abis/0.3.0+gitrAUTOINC+b6d2834eef-r2.14.0/git/src/input/dahdi.c:25:26: fatal error: ../../config.h: No such file or directory
 #include "../../config.h"
diff --git a/src/input/dahdi.c b/src/input/dahdi.c
index 030bef3..9f1f2bb 100644
--- a/src/input/dahdi.c
+++ b/src/input/dahdi.c
@@ -22,7 +22,7 @@
  *
  */
 
-#include "../../config.h"
+#include "config.h"
 
 #ifdef HAVE_DAHDI_USER_H