Fix all references to config.h

config.h is created in $(top_buildir)/config.h.
Let's make sure all CPPFLAGS add correct -Ipath includes,
and that all code includes the correct file.

Change-Id: Ie9ea38bb009bc715b01cde4d66d181f7bec2e7bd
diff --git a/src/core/socket.c b/src/core/socket.c
index ee49c27..43736e0 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -17,7 +17,7 @@
  *
  */
 
-#include "../config.h"
+#include "config.h"
 
 /*! \addtogroup socket
  *  @{