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/tests/socket/socket_test.c b/tests/socket/socket_test.c
index b3b9584..ddb6926 100644
--- a/tests/socket/socket_test.c
+++ b/tests/socket/socket_test.c
@@ -32,7 +32,7 @@
 #include <osmocom/core/logging.h>
 #include <osmocom/core/bits.h>
 
-#include "../config.h"
+#include "config.h"
 
 void *ctx = NULL;