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/plugin.c b/src/core/plugin.c
index 5f44a40..687ad40 100644
--- a/src/core/plugin.c
+++ b/src/core/plugin.c
@@ -23,7 +23,7 @@
  *  @{
  * \file plugin.c */
 
-#include "../config.h"
+#include "config.h"
 
 #if HAVE_DLFCN_H